workflows: use actions/checkout@v3

Use the new version of actions/checkout, since all Github Actions will
begin running on Node16 instead of Node12.

See https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/.

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
       matrix:
         zig: [ 0.6.0, 0.7.0, 0.8.0, 0.9.0, 0.10.0 ]
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
 
     - name: Setup Zig
       uses: goto-bus-stop/setup-zig@v2

.github/workflows/eowyn.yml

@@ -18,7 +18,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     timeout-minutes: 60
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Setup Zig
       uses: goto-bus-stop/setup-zig@v2
       with: