Update actions/checkout action to v3

.github/workflows/test.yml

@@ -6,7 +6,7 @@ jobs:
   test-nix:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
       with:
           # Nix Flakes doesn't work on shallow clones
           fetch-depth: 0
@@ -29,7 +29,7 @@ jobs:
       with:
         go-version: ${{ matrix.go-version }}
     - name: Checkout code
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
     - name: Format
       run: if [ "$(gofmt -s -l . | wc -l)" -gt 0 ]; then exit 1; fi
       if: matrix.os == 'ubuntu-latest'