simple first workflow checking out postgres on osx

.github/workflows/build-os-x.yaml

@@ -13,7 +13,7 @@ jobs:
       - name: Check out postgres code
         uses: actions/checkout@master
         with:
-          repository: https://github.com/postgres/postgres.git
+          repository: postgres/postgres
           ref: refs/tags/REL_16_2
           fetch-tags: true
       - run: echo "💡 The postgres repository has been cloned to the runner."