Move newsfragment check to a separate workflow (#1193)

... to make sure we still get a preview even if there is no newsfragment.

.github/workflows/checks.yaml

@@ -0,0 +1,18 @@
+# workflow steps that ought to pass on a PR, but shouldn't block a preview.
+
+name: "Checks"
+on:
+  pull_request:
+ 
+jobs:
+  check-newsfragments:
+    name: "🔎 Check that new newsfragments are valid"
+    if: github.event_name == 'pull_request'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - run: scripts/check-newsfragments
+        env:
+          PULL_REQUEST_NUMBER: ${{ github.event.number }}

.github/workflows/main.yml

@@ -45,18 +45,6 @@ jobs:
         run: |
           /env/bin/python scripts/check-event-schema-examples.py
         
-  check-newsfragments:
-    name: "🔎 Check that new newsfragments are valid"
-    if: github.event_name == 'pull_request'
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v2
-        with:
-          fetch-depth: 0
-      - run: scripts/check-newsfragments
-        env:
-          PULL_REQUEST_NUMBER: ${{ github.event.number }}
-        
   calculate-baseurl:
     name: "⚙️ Calculate baseURL for later jobs"
     runs-on: ubuntu-latest