fix(setup-osuosl-ca): use pwsh on Windows to avoid Cygwin CRLF issue

[bastelfreak]

The composite action used 'shell: bash' for all platforms. On windows runners with Cygwin, GitHub Actions generates the temp script with Windows CRLF line endings. Cygwin bash without '-o igncr' treats the \r as a command name, resulting in exit code 127.

We switch to PowerShell on windows to use native tools instead of Cygwin.
Fixes: https://github.com/OpenVoxProject/puppet-runtime/actions/runs/33179641401/job/98877289758

Error for future reference:

  # Replace any backslashes with forward slashes so that a Windows path
  # is usable from both POSIX and WIN32 contexts.
  osuosl_ca="${RUNNER_TEMP//\\//}/osuosl-ca.pem"

  printf 'Downloading Mozilla CA bundle to: %s\n' "${osuosl_ca}"

  curl -sSL https://curl.se/ca/cacert.pem -o "${osuosl_ca}"

  printf 'AWS_CA_BUNDLE=%s' "${osuosl_ca}" >>$GITHUB_ENV
  shell: C:\cygwin64\bin\bash.EXE --noprofile --norc -e -o pipefail {0}
  env:
    VANAGON_LOCATION: https://github.com/openvoxproject/vanagon#main
D:\a\_temp\dcb0f279-a648-4603-9fd7-75c1faa0e719.sh: line 4: $'\r': command not found
Error: Process completed with exit code 127.

Short description

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• read and accepted the AI Policy document and added Generated-by or Assisted-by annotations to each of my commits created with the help of an AI agent
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[bastelfreak]

The commit description is my guess. I had no time yet to test it.

[bastelfreak]

Test: https://github.com/OpenVoxProject/puppet-runtime/actions/runs/33199075318

[bastelfreak]

https://github.com/OpenVoxProject/puppet-runtime/actions/runs/33199075318/job/98943704496 works fine \o/