CI: activate github actions on 1.1.x (PR only) (#35467)

Author: simonjayhawkins

.github/workflows/ci.yml

@@ -4,7 +4,9 @@ on:
   push:
     branches: master
   pull_request:
-    branches: master
+    branches:
+      - master
+      - 1.1.x
 
 env:
   ENV_FILE: environment.yml