Update ci.yml (#98)

Warning when using v2:
Node.js 12 actions are deprecated.

v3 uses node16 runtime by default.

Author: nkalvi

.github/workflows/ci.yml

@@ -25,6 +25,6 @@ jobs:
     name: Ubuntu
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Run tests
       run: make test-linux