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

@@ -18,7 +18,7 @@ jobs:
       linux_build_command: 'swift test --no-parallel'
       linux_swift_versions: '["nightly-main", "nightly-6.2"]'
       windows_swift_versions: '["nightly-main"]'
-      windows_build_command: 'swift test --no-parallel'
+      windows_build_command: 'Invoke-Program swift test --no-parallel'
       enable_linux_static_sdk_build: true
       linux_static_sdk_build_command: SWIFTBUILD_STATIC_LINK=1 LLBUILD_STATIC_LINK=1 swift build
   cmake-smoke-test: