Update GHA checkout action to v3

This fixes a deprecation warning

Author: bjorn3

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 10
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - name: Test
         run: |
           cargo test
@@ -18,7 +18,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 10
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - name: Install rustfmt
         run: |
           rustup component add rustfmt