ci: add support for gitlab-runner 'shell = pwsh' executor

Run powershell child scripts using the same powershell tool as the
parent script.

.gitlab/ci/env.ps1

@@ -1,3 +1,4 @@
+$pwsh = [System.Diagnostics.Process]::GetCurrentProcess().MainModule.FileName
 if (Test-Path -Path ".gitlab/ci/env_$env:CMAKE_CONFIGURATION.ps1" -PathType Leaf) {
   . ".gitlab/ci/env_$env:CMAKE_CONFIGURATION.ps1"
 }

.gitlab/ci/env_windows_vs2022_x64_ninja_multi.ps1

@@ -3,7 +3,7 @@ if ("$env:CMAKE_CI_NIGHTLY" -eq "true") {
 }
 
 $pwdpath = $pwd.Path
-powershell -File ".gitlab/ci/ninja.ps1"
+& "$pwsh" -File ".gitlab/ci/ninja.ps1"
 Set-Item -Force -Path "env:PATH" -Value "$pwdpath\.gitlab;$env:PATH"
 ninja --version
 

.gitlab/ci/ispc-env.ps1

@@ -1,4 +1,4 @@
 $pwdpath = $pwd.Path
-powershell -File ".gitlab/ci/ispc.ps1"
+& "$pwsh" -File ".gitlab/ci/ispc.ps1"
 Set-Item -Force -Path "env:PATH" -Value "$pwdpath\.gitlab\ispc\bin;$env:PATH"
 ispc --version

.gitlab/os-windows.yml

@@ -295,11 +295,11 @@
 .before_script_windows: &before_script_windows
     - . .gitlab/ci/env.ps1
     - $pwdpath = $pwd.Path
-    - powershell -File ".gitlab/ci/wix.ps1"
+    - (& "$pwsh" -File ".gitlab/ci/wix.ps1")
     - Set-Item -Force -Path "env:WIX" -Value "$pwdpath\.gitlab\wix"
-    - powershell -File ".gitlab/ci/cmake.ps1"
+    - (& "$pwsh" -File ".gitlab/ci/cmake.ps1")
     - Set-Item -Force -Path "env:PATH" -Value "$pwdpath\.gitlab\cmake\bin;$env:PATH"
-    - powershell -File ".gitlab/ci/ninja.ps1"
+    - (& "$pwsh" -File ".gitlab/ci/ninja.ps1")
     - Set-Item -Force -Path "env:PATH" -Value "$pwdpath\.gitlab;$env:PATH"
     - (& "$env:WIX\bin\light.exe" -help) | Select -First 1
     - cmake --version
@@ -398,7 +398,7 @@
     script:
         - . .gitlab/ci/env.ps1
         - $pwdpath = $pwd.Path
-        - powershell -File ".gitlab/ci/ninja.ps1"
+        - (& "$pwsh" -File ".gitlab/ci/ninja.ps1")
         - Set-Item -Force -Path "env:PATH" -Value "$pwdpath\.gitlab;$env:PATH"
         - Invoke-Expression -Command .gitlab/ci/clang.ps1
         - Invoke-Expression -Command .gitlab/ci/vcvarsall.ps1