GitHub Actions: updade checkout Action to v3

I'm not sure what this step actually does since there is no `with:` section.

.github/workflows/buildmaster.yml

@@ -23,7 +23,7 @@ jobs:
 
     steps:
     - name: Checkout master
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
 
     - name: Setup build environment
       run: |