Use Invoke-Program for the Windows build command

Windows does not stop scripts when native commands exit with non-zero.
Instead, their exit status has to be checked manually. This can be done
through an `Invoke-Program` function that is added to the script running
`windows_build_command`, which also previously prefixed the given
command. This is changing in
swiftlang/github-workflows#154 since it doesn't
help with multi-line commands - update our modified
`windows_build_command` to use `Invoke-Program` instead.

Author: bnbarham

.github/workflows/nightly.yml

@@ -68,7 +68,7 @@ jobs:
         VSCODE_SWIFT_VSIX_ID=${{needs.package.outputs.artifact-id}}
         GITHUB_REPOSITORY=${{github.repository}}
       windows_pre_build_command: . .github\workflows\scripts\windows\setup.ps1
-      windows_build_command: scripts\test_windows.ps1
+      windows_build_command: Invoke-Program scripts\test_windows.ps1
       enable_windows_docker: false
 
   tests_insiders:
@@ -99,5 +99,5 @@ jobs:
         VSCODE_SWIFT_VSIX_PRERELEASE=1
         GITHUB_REPOSITORY=${{github.repository}}
       windows_pre_build_command: . .github\workflows\scripts\windows\setup.ps1
-      windows_build_command: scripts\test_windows.ps1
+      windows_build_command: .github/workflows/nightly.yml scripts\test_windows.ps1
       enable_windows_docker: false