gitlab-ci: Add wix, cmake, and ninja to PATH incrementally on Windows

Also run their scripts in an isolated child process.

.gitlab/os-windows.yml

@@ -151,13 +151,14 @@
 ## Windows-specific scripts
 
 .before_script_windows: &before_script_windows
-    - Invoke-Expression -Command .gitlab/ci/wix.ps1
-    - Invoke-Expression -Command .gitlab/ci/cmake.ps1
-    - Invoke-Expression -Command .gitlab/ci/ninja.ps1
     - $pwdpath = $pwd.Path
+    - powershell -File ".gitlab/ci/wix.ps1"
     - Set-Item -Force -Path "env:WIX" -Value "$pwdpath\.gitlab\wix"
+    - powershell -File ".gitlab/ci/cmake.ps1"
+    - Set-Item -Force -Path "env:PATH" -Value "$pwdpath\.gitlab\cmake\bin;$env:PATH"
+    - powershell -File ".gitlab/ci/ninja.ps1"
+    - Set-Item -Force -Path "env:PATH" -Value "$pwdpath\.gitlab;$env:PATH"
     - (& "$env:WIX\bin\light.exe" -help) | Select -First 1
-    - Set-Item -Force -Path "env:PATH" -Value "$pwdpath\.gitlab;$pwdpath\.gitlab\cmake\bin;$env:PATH"
     - cmake --version
     - ninja --version
     - cmake -P .gitlab/ci/download_qt.cmake