The ParsedURL url field was dropped in Nix commit
f705ce7f9a95bc37df236d88eef1dbd9e2ae5f31, which says to use to_string()
instead.

As of Nix commit beed00c04e136e8d685905e4b2b1116ecdf42f63, absPath takes
a string_view

InputPath was renamed to InputAttrPath in Nix commit
7dfff58292475d0aed3dc3e98236ba495c45f261

As of Nix commit 7b6622d73317f0cd093cde96b5898b8af07abea7, a NixInt is
no longer just a typedef so we have to use .value to extract an integer

We should only be using paths from nix.out, not nix.dev. Removing
nix.dev from propagatedBuildInputs reduces our closure size by ~60 MiB.
Before this change, on aarch64-linux:

```
$ nix path-info --size --closure-size --human-readable ./result
/nix/store/58yi21g394bzy5zdg0jzar7x1kg5cp9x-flox-1.4.3-g59e52f0	  82.8 KiB	 373.2 MiB
```

After:
```
$ nix path-info --size --closure-size --human-readable ./result
/nix/store/nwk9pqbxjb37hcwxjd3l5g18g2ig1lmj-flox-1.4.3-g3bb091d	  82.8 KiB	 315.9 MiB
```

I increased large-error-threshold for now to silence some lints, but we
can fix those if we want as followup

Add a newline in expected test output. The previous behavior was
incorrect and logged in NixOS/nix#11991 which
has since been fixed.

Updating our flake increases closure size on aarch64-linux from 294.8
MiB to 315.9 MiB.

The closure size of Nix from 2.24.12 to 2.28.2 went up from 108.4 to
131.1 MiB.

As this is a necessary dependency, increase closure limit size
accordingly.

It does look like Nix may have introduced some unnecessary dependencies,
but if we think those are worth chasing down we can do it in followup.

Bump testing nixpkgs URL to stay in sync with the nixpkgs we use to
build Flox.

If we don't do this, it will cause build tests to fail, as certain build
tests currently depend on the fact that patchShebang will patch using a
bash that is in the closure of flox-interpreter. If the nixpkgs used for
patchShebang is different than the one used to build flox-interpreter,
the build will fail with errors like:
```
❌ 1 packages found in /nix/store/sg8yy9rf2wi4jakxjdfq7ly220647wqh-mypkg-1.0.2a
   -      not found in /nix/store/4w3fvpfl2yjabdb536ij74zlp8hwwz1l-environment-build-mypkg
```

opt-level > 0 causes hyper to create bad requests because of a rustc
bug, so set opt-level to 0

I also left .connection_verbose(true) in our client builder as it was
helpful during debugging.