Configure CI for merge_queue.

Fixes #742.

See https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue#triggering-merge-group-checks-with-github-actions

.github/workflows/ci.yml

@@ -2,10 +2,7 @@ name: CI
 
 on:
   pull_request:
-  push:
-    branches:
-      - staging
-      - trying
+  merge_group:
 
 jobs:
   check:
@@ -51,13 +48,3 @@ jobs:
           toolchain: stable
           components: rustfmt
       - run: cargo fmt --check
-
-  # https://github.com/rust-lang/crater/blob/9ab6f9697c901c4a44025cf0a39b73ad5b37d198/.github/workflows/bors.yml#L125-L149
-  end-success:
-    name: bors build finished
-    if: success()
-    runs-on: ubuntu-latest
-    needs: [check, test]
-    steps:
-      - name: Mark the job as successful
-        run: exit 0