Fix generated PowerShell init script

[flandia]

Problem

Since #5864, opam has generated the following PowerShell:

if Test-Path "...\variables.ps1" { ... }

PowerShell requires parentheses around an if condition and rejects this
script with:

Missing '(' after 'if' in if statement.

Fix

Generate valid PowerShell syntax:

if (Test-Path "...\variables.ps1") { ... }

The Windows reftest now executes the generated script using Windows
PowerShell, ensuring future syntax errors fail the test.

Testing

dune build --profile=release --root . '@reftest-init-scripts.win32' --force

The generated script was also executed successfully with Windows PowerShell
5.1 and PowerShell 7.

[kit-ty-kate]

neat idea to at least test the syntax, it should probably be done for every shell we support (you're welcome to do it but it's not strictly necessary for this PR so no pressure)

[flandia]

I tried to add them but I'm not sure if it works. Would you approve the workflow and test it?

[kit-ty-kate]

done

[flandia]

The workflow failed and I guess that's due to an upstream problem which has been fixed. I updated the branch. Would you approve the workflow again? Thanks

[kit-ty-kate]

For the record, you can open a PR on your own fork and enable GitHub Action on it, if you want to debug something. This way i don't have to approve it every time

[flandia]

Thanks for the tip. The checks succeeded. If maintainers do not have further comments, I believe this PR is now ready to merge.