chore: upgrade to actions/checkout@v3

Author: wdconinc

.github/workflows/cvmfs_http_proxy.yml

@@ -4,7 +4,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - uses: ./
       with:
         cvmfs_http_proxy: 'auto'