Enable macOS CI on pull requests (#64)

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

@@ -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-http-structured-headers-Package

.github/workflows/pull_request.yml

@@ -23,3 +23,10 @@ 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-http-structured-headers-Package