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/New-BuildVM.ps1

@@ -31,6 +31,13 @@ $pyArgs = @(
 )
 
 $venvPython = $env:CI_VENV_PYTHON
-if (-not $venvPython) { $venvPython = 'F:\CI\python\venv\Scripts\python.exe' }
+if (-not $venvPython) {
+    # $IsWindows exists only on PS 6+; absent on PS 5.1 (always Windows)
+    if ($null -ne $IsWindows -and $IsWindows -eq $false) {
+        $venvPython = '/opt/ci/venv/bin/python'
+    } else {
+        $venvPython = 'F:\CI\python\venv\Scripts\python.exe'
+    }
+}
 & $venvPython -m ci_orchestrator vm new @pyArgs
 exit $LASTEXITCODE