Enable macOS CI on pull requests (#170)

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.

.github/workflows/main.yml

@@ -21,4 +21,5 @@ jobs:
     name: macOS tests
     uses: apple/swift-nio/.github/workflows/macos_tests.yml@main
     with:
+      runner_pool: nightly
       build_scheme: swift-metrics-Package

.github/workflows/pull_request.yml

@@ -24,3 +24,10 @@ jobs:
   cxx-interop:
     name: Cxx interop
     uses: apple/swift-nio/.github/workflows/cxx_interop.yml@main
+
+  macos-tests:
+    name: macOS tests
+    uses: apple/swift-nio/.github/workflows/macos_tests.yml@main
+    with:
+      runner_pool: general
+      build_scheme: swift-metrics-Package