github: add merge_group event to several workflows

These are the workflows that run on PRs, so they need to have the `merge_group`
event added to them if we want to use the merge queue.

Signed-off-by: Austin Seipp <aseipp@pobox.com>

.github/workflows/build-nix.yml

@@ -5,6 +5,7 @@ on:
     branches:
       - main
   pull_request:
+  merge_group:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}

.github/workflows/build.yml

@@ -3,6 +3,7 @@ name: build
 on:
   push:
   pull_request:
+  merge_group:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}

.github/workflows/codespell.yml

@@ -5,6 +5,7 @@ on:
     branches:
       - main
   pull_request:
+  merge_group:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}

.github/workflows/dependabot.yml

@@ -2,6 +2,7 @@ name: Enable auto-merge for Dependabot PRs
 
 on:
   pull_request:
+  merge_group:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}