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/pull_request.yml

@@ -11,7 +11,7 @@ jobs:
     with:
       linux_build_command: "swift build"
       linux_exclude_swift_versions: '[{"swift_version": "5.8"}, {"swift_version": "5.9"}, {"swift_version": "5.10"}]'
-      windows_build_command: "swift build"
+      windows_build_command: "Invoke-Program swift build"
       windows_exclude_swift_versions: '[{"swift_version": "5.9"}, {"swift_version": "5.10"}]'
       enable_macos_checks: true
       macos_xcode_versions: '["16.3"]'