Enable macOS CI on pull requests

Motivation:

* Improve test coverage

Modifications:

Enable macOS CI to be run on pull request commits and make the use of the nightly runner pool for main.yml jobs explicit.

Result:

Improved test coverage.

Author: rnro

.github/workflows/main.yml

@@ -56,4 +56,5 @@ jobs:
     name: macOS tests
     uses: apple/swift-nio/.github/workflows/macos_tests.yml@main
     with:
+      runner_pool: nightly
       build_scheme: swift-nio-http2-Package