Extracted code linting test to separate CI job

- this was taking over a minute to run (?!), and was running on each
  matrix job
- we only need to run it once as changing Node versions shouldn't change
  the results

.github/workflows/test.yml

@@ -10,6 +10,24 @@ on:
 env:
   FORCE_COLOR: 1
 jobs:
+  lint:
+    runs-on: ubuntu-18.04
+    if: github.event_name == 'push' || (github.event_name == 'pull_request' && !startsWith(github.head_ref, 'renovate/'))
+    name: Lint
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-node@v1
+        with:
+          node-version: '12.10.0'
+      - run: yarn
+      - run: yarn lint
+      - uses: daniellockyer/action-slack-build@master
+        if: failure() && github.event_name == 'push' && github.ref == 'refs/heads/master'
+        with:
+          status: ${{ job.status }}
+        env:
+          SLACK_WEBHOOK_URL: ${{ secrets.SLACK_WEBHOOK_URL }}
+
   test:
     runs-on: ubuntu-18.04
     if: github.event_name == 'push' || (github.event_name == 'pull_request' && !startsWith(github.head_ref, 'renovate/'))
@@ -44,7 +62,6 @@ jobs:
           mysql root password: 'root'
 
       - run: yarn
-      - run: yarn lint
       - run: grunt test-acceptance --verbose
       - run: grunt test-unit --verbose
       - run: grunt test-regression --verbose