CD: Run one job per repo at a time

Recently started deploying to two apps in the same repository[1], which
causes a race condition as the two deploys start at the same time, but
need to be ordered to avoid conflicts[2].

[1] 25f7cc3
[2] https://github.com/navapbc/template-application-rails/actions/runs/14063004743/attempts/1

Author: doshitan

.github/workflows/cd.yml

@@ -23,6 +23,8 @@ jobs:
             app: paidleave
           - repo: navapbc/pfml-starter-kit-app
             app: demo
+    concurrency:
+      group: cd-${{ matrix.project.repo }}
     steps:
       - name: Checkout project repo
         uses: actions/checkout@v4