Update actions/checkout action to v3

Author: renovate-bot

.github/workflows/mix_test.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-latest
     name: mix test
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: cachix/install-nix-action@v17
       - uses: cachix/cachix-action@v10
         with: