Run CI on merge queue things (#1658)

Should unblock our merge queues.

---

<!-- Consider each and tick it off one way or the other -->
* [ ] CHANGELOG updated or not needed
* [ ] Documentation updated or not needed
* [ ] Haddocks updated or not needed
* [ ] No new TODOs introduced or explained herafter

.github/workflows/ci-nix.yaml

@@ -4,8 +4,9 @@ name: "CI"
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
   cancel-in-progress: true
-
 on:
+  # We're using merge-chains; so this needs to run then.
+  merge_group:
   push:
     branches:
     - master

.github/workflows/formatting.yaml

@@ -1,6 +1,8 @@
 name: Check code formatting
 
 on:
+  # We're using merge-chains; so this needs to run then.
+  merge_group:
   pull_request:
 
 jobs:

.github/workflows/network-test.yaml

@@ -1,6 +1,8 @@
 name: "Network fault tolerance"
 
 on:
+  # We're using merge-chains; so this needs to run then.
+  merge_group:
   push:
     branches:
     - master