fix(shims): add Linux fallback for venv Python path in all PS shims

All shims defaulted to F:\CI\python\venv\Scripts\python.exe when
CI_VENV_PYTHON was not set. On Linux host this caused instant failure.

Now uses $IsWindows (PS 6+ automatic var; absent on PS 5.1 = Windows)
to select /opt/ci/venv/bin/python on Linux. Windows paths unchanged.

Affects: Invoke-CIJob, Get-BuildArtifacts, Cleanup-OrphanedBuildVMs,
New-BuildVM, Remove-BuildVM, Wait-VMReady, Get-CIJobSummary,
Invoke-RemoteBuild, Watch-DiskSpace, Watch-RunnerHealth,
Set-CIGuestCredential, Test-CIGuestWinRM.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/Set-CIGuestCredential.ps1

@@ -68,6 +68,7 @@ param(
     [System.Security.SecureString] $Password,
 
     [string] $VenvPython = $(if ($env:CI_VENV_PYTHON) { $env:CI_VENV_PYTHON }
+                            elseif ($null -ne $IsWindows -and $IsWindows -eq $false) { '/opt/ci/venv/bin/python' }
                             else { 'F:\CI\python\venv\Scripts\python.exe' }),
 
     [switch] $SkipVerify