Use master checkout (#53)

Author: ad-m

.github/workflows/build.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@master
     - name: Verify action syntax 
       # The action should not publish any real changes, but should succeed.
       uses: './'