Update nightly smoke test for macOS to use the nightly toolchain (#372)

Update nightly smoke test for macOS to use the nightly toolchain for build/test step

Author: cmcgee1024

.github/workflows/nightly_snapshot_check.yml

@@ -36,4 +36,4 @@ jobs:
       - name: Prepare the action
         run: ./scripts/prep-gh-action.sh --install-swiftly --swift-main-snapshot
       - name: Build and Test
-        run: swift test
+        run: swiftly run +main-snapshot swift test