Pin version to v2 for actions/checkout in CI workflow (#45)

Before `actions/checkout` was used from the `master` branch [1] in the
`ci` workflow [2]. This has been changed to the pinned version `v2`
to ensure a stable pipline.

[1]: https://github.com/svengreb/tmpl/blob/e53fbaf8ff974a7c61d1ff51602175c82a35b20e/.github/workflows/ci.yml#L31
[2]: https://github.com/svengreb/tmpl/actions?query=workflow%3Aci

Closes GH-44

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ on:
       - v*
   pull_request:
 jobs:
-  lint:
+  lint-node:
     runs-on: ubuntu-latest
     strategy:
       matrix:
@@ -28,7 +28,7 @@ jobs:
           echo "Workflow Actor: $GITHUB_ACTOR"
           echo "GitHub Action Runner: $TMPL_GITHUB_ACTION_CONTEXT_RUNNER"
       - name: Checkout repository
-        uses: actions/checkout@master
+        uses: actions/checkout@v2
       - name: "Setup Node.js version ${{ matrix.node-version }}"
         uses: actions/setup-node@v1
         with: