workflows/tests: enable concurrency limiting.

Let's cancel in-progress builds on the same PR.

.github/workflows/tests.yml

@@ -1,12 +1,19 @@
 name: CI
+
 on:
   push:
     branches:
       - master
   pull_request:
+
 env:
   HOMEBREW_DEVELOPER: 1
   HOMEBREW_NO_AUTO_UPDATE: 1
+
+concurrency:
+  group: "${{ github.ref }}"
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 jobs:
   syntax:
     if: github.repository == 'Homebrew/brew'