Use master checkout (#53)

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index d4cedd3..a1ec4d7 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -9,7 +9,7 @@
     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: './'