Edit required check so that it doesn't hang when only non-code files modified

.github/workflows/build-linux.yml

@@ -1,11 +1,9 @@
 name: Linux CI
 
-on:
-  pull_request:
-    paths-ignore:
-      - '**.md'
-      - '**.ipynb' 
-      - 'myst.yml'
+# Since this is a required check, specify paths-ignore in the check-paths job
+# instead of under 'pull_request:'. Otherwise, the check is still required but
+# never runs, and a maintainer must bypass the check in order to merge the PR.
+on: [pull_request]
 
 # Every time you make a push to your PR, it cancel immediately the previous checks,
 # and start a new one. The other runner will be available more quickly to your PR.
@@ -14,7 +12,26 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  # Check paths to changed files to see if any are non-ignored.
+  check-paths:
+    runs-on: ubuntu-latest
+    outputs:
+      should_run: ${{ steps.filter.outputs.changes_detected }}
+    steps:
+      - name: Check modified files
+        id: filter
+        uses: dorny/paths-filter@v2
+        with:
+          filters: |
+            ignored:
+              - '!**.md'
+              - '!**.ipynb' 
+              - '!myst.yml'
   build:
+    # Only run build if relevant files have been modified in this PR.
+    needs: check-paths
+    if: needs.check-paths.outputs.should_run == 'true'
+
     name: ${{ matrix.name }} ${{ matrix.build_type }}
     runs-on: ${{ matrix.os }}