Add a merge_group trigger to the build workflow

This should improve compatibility with merge queues. We configure it to
only trigger on the `checks_requested` type which is currently the only
supported type for this trigger. If additional types are added in the
future they should be added if appropriate.

.github/workflows/build.yml

@@ -2,6 +2,9 @@
 name: build
 
 on:
+  merge_group:
+    types:
+      - checks_requested
   pull_request:
   push:
   repository_dispatch: