This is to address 3 related review comments at once:
- #958 (comment)
- #958 (comment)
- #958 (comment)

It consists in moving the Zsh setup close to its Nushell peer, that is
prior to installing helper scripts.

This is to comply with
#958 (comment),
as it turns out local code owners prefer using "$HOME" over ~, despite
the guaranteed tilde expansion in POSIX Shells:
- https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_01
- https://www.gnu.org/software/bash/manual/html_node/Tilde-Expansion.html#Tilde-Expansion-1

It turns out `bind` mounts introduced by #958 in Linux Docker builds
also work well in Windows builds.

The present change therefore proposes to leverage the feature here too
so as to shrink down layers affected by repeated `COPY`s. (overlays)

Doing so unearthed a few issues: (thanks to `bind` mounts in builds
being read/only by design)
- quite a number of temporary files (installers, tarballs) where
  downloaded/extracted to various depths within installation scripts,
- some of these temporaries were not removed after installation:
  - under `./`: `get-pip.py`, (first occurrence)
  - under `./helpers/phase1`: `wdksetup.exe`,
  - under `./helpers/phase2`: `get-pip.py`, (second occurrence)
  - under `./helpers/phase3`: `rustup-init.exe`.

The present change therefore addresses these as well.

It turns out `bind` mounts introduced by #958 in Linux Docker builds
also work well in Windows builds.

The present change therefore proposes to leverage the feature here too
so as to shrink down layers affected by repeated `COPY`s. (overlays)

Doing so unearthed a few issues: (thanks to `bind` mounts in builds
being read/only by design)
- quite a number of temporary files (installers, tarballs) where
  downloaded/extracted to various depths within installation scripts,
- some of these temporaries were not removed after installation:
  - under `./`: `get-pip.py`, (first occurrence)
  - under `./helpers/phase1`: `wdksetup.exe`,
  - under `./helpers/phase2`: `get-pip.py`, (second occurrence)
  - under `./helpers/phase3`: `rustup-init.exe`.

The present change therefore addresses these as well.

It turns out `bind` mounts introduced by #958 in Linux Docker builds
also work well in Windows builds.

The present change therefore proposes to leverage the feature here too
so as to shrink down layers affected by repeated `COPY`s. (overlays)

Doing so unearthed a few issues: (thanks to `bind` mounts in builds
being read/only by design)
- quite a number of temporary files (installers, tarballs) where
  downloaded/extracted to various depths within installation scripts,
- some of these temporaries were not removed after installation:
  - under `./`: `get-pip.py`, (first occurrence)
  - under `./helpers/phase1`: `wdksetup.exe`,
  - under `./helpers/phase2`: `get-pip.py`, (second occurrence)
  - under `./helpers/phase3`: `rustup-init.exe`.

The present change therefore addresses these as well.

It turns out `bind` mounts introduced by #958 in Linux Docker builds
also work well in Windows builds.

The present change therefore proposes to leverage the feature here too
so as to shrink down layers affected by repeated `COPY`s. (overlays)

Doing so unearthed a few issues: (thanks to `bind` mounts in builds
being read/only by design)
- quite a number of temporary files (installers, tarballs) where
  downloaded/extracted to various depths within installation scripts,
- some of these temporaries were not removed after installation:
  - under `./`: `get-pip.py`, (first occurrence)
  - under `./helpers/phase1`: `wdksetup.exe`,
  - under `./helpers/phase2`: `get-pip.py`, (second occurrence)
  - under `./helpers/phase3`: `rustup-init.exe`.

The present change therefore addresses these as well.

* Also use `bind` mounts in Windows Docker builds

It turns out `bind` mounts introduced by #958 in Linux Docker builds
also work well in Windows builds.

The present change therefore proposes to leverage the feature here too
so as to shrink down layers affected by repeated `COPY`s. (overlays)

Doing so unearthed a few issues: (thanks to `bind` mounts in builds
being read/only by design)
- quite a number of temporary files (installers, tarballs) where
  downloaded/extracted to various depths within installation scripts,
- some of these temporaries were not removed after installation:
  - under `./`: `get-pip.py`, (first occurrence)
  - under `./helpers/phase1`: `wdksetup.exe`,
  - under `./helpers/phase2`: `get-pip.py`, (second occurrence)
  - under `./helpers/phase3`: `rustup-init.exe`.

The present change therefore addresses these as well.

* Map `WORKDIR` to a meaningful directory

This is to address following review comment:
> Might be better to use Windows-style paths here, maybe `c:/mnt` ?
(#963 (comment))

The idea here is to map WORKDIR to a well-known, existing root directory
(the default one under Windows).

For good measure, the earlier mount point gets also deleted.

### What does this PR do?

Replaces all `RUN --mount` instructions in the Dockerfiles to specifically mount the files it needs instead of the entire repo.

Also moves a bunch of logic from the dockerfile itself into separate scripts for better readability and maintainability.

### Motivation

Mounting the entire repo _destroys_ caching performance: with the entire repo mounted, the cached value of each layer is invalidated as soon as _any_ file in the repo gets changed.

By mounting only the necessary file(s) in each `RUN` layer, we make sure that these layer's cached values are invalidated if and only if these specific files get changed, instead of any file in the repo.

### Possible Drawbacks / Trade-offs

None, as long as every image builds properly. Maybe it's a bit more verbose.

### Additional Notes

See #958.
I did not make the change for the Windows image as it requires mounting a lot more files - I will probably do it in a follow-up PR. 


Tested in DataDog/datadog-agent#41927