PS: Run tests on PRs

[MathiasVP]

This modifies the existing powershell-pr-check workflow so that, instead of just compiling the queries in powershell/ql/src it'll run all the powershell tests in powershell/ql/test.

Additionally there's a list of changes I had to do:

• Since the build script is a .ps1 file I had to change the workflow to run on windows-latest instead of ubuntu-latest. I assume testing on Windows is actually more reflective of the production environment, although we could also add an Ubuntu version if we added a non-.ps1 build script
• I hardened the build script so that it's agnostic to where the script is run from