CI: Run lint with Alchemy 7.3

The main branch (which is the default branch if the ALCHEMY_BRANCH
env is not set) does not work with this Gem yet. We do not need
the latest version of Alchemy for the linting to work.

Author: tvdeyen

.github/workflows/lint.yml

@@ -9,6 +9,8 @@ concurrency:
 jobs:
   Standard:
     runs-on: ubuntu-latest
+    env:
+      ALCHEMY_BRANCH: 7.3-stable
     steps:
       - name: Checkout code
         uses: actions/checkout@v4