Upgrade checkout action to v3 to resolve dubious ownership error

See these issues for more info, v3 automatically sets repo as safe directory:
* actions/checkout#1048
* actions/runner-images#6775

.github/workflows/pkg.yml

@@ -31,7 +31,7 @@ jobs:
           yum install --assumeyes https://packages.endpointdev.com/rhel/7/os/x86_64/endpoint-repo.x86_64.rpm
           yum install --assumeyes git gcc
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
       - name: Init Hermit
         run: ./bin/hermit env --raw >> $GITHUB_ENV
       - name: Build