Fixes #19099
Supersedes #18384

### Summary of the issue:

Some files are not signed some of the time

### Description of user facing changes:

None

### Description of developer facing changes:

In the case of alpha, beta, rc, stable and try builds, failure to sign
the launcher or most of our executables will fail the build.
All CI builds now run serially to avoid race conditions.

### Description of development approach:

Set the `ErrorAction` common parameter on `Send-SigningRequest` to
`Stop` to cause the signing script to fail if the cmd-let fails.
Use `Get-AuthenticodeSignature` to verify the signature of files after
`Submit-SigningRequest` returns.

### Testing strategy:

CI

* [x] Successful signing:
https://github.com/nvaccess/nvda/actions/runs/18733609673
  * Note that this run's failure was due to a system test failure
* Checked that the launcher and all `dll` and `exe` files are signed.
Found unsigned `exe`s and `dll` s by unzipping the controller client and
launcher into the same directory, and running:
    
    ```ps1
(Get-ChildItem -Recurse -Include *.exe, *.dll -Name |
Get-AuthenticodeSignature | where-object {$_.Status -ne 'Valid'}).Path
    ```

    The following files do not have valid signatures:

    * `app\brailleDisplayDrivers\lilli.dll`
    * `app\miscDeps\tools\msgfmt.exe`
    * `app\synthDrivers\espeak.dll`
    * `app\synthDrivers\sonic.dll`
    * `app\brlapi-0.8.dll`
    * `app\libgcc_s_dw2-1.dll`
    * `app\wxbase32u_net_vc140.dll`
    * `app\wxbase32u_vc140.dll`
    * `app\wxmsw32u_aui_vc140.dll`
    * `app\wxmsw32u_core_vc140.dll`
    * `app\wxmsw32u_html_vc140.dll`
    * `app\wxmsw32u_stc_vc140.dll`
    * `Banner.dll`
    * `System.dll`

However, as best as I can tell, we never attempt to sign these files.

* [x] Intentionally don't sign a DLL:
https://github.com/nvaccess/nvda/actions/runs/18703904287
* [x] Intentionally don't sign the launcher:
https://github.com/nvaccess/nvda/actions/runs/18707118372

### Known issues with pull request:

None