CI: use concurrency groups to make sure that we only run one integrat…

…ion test job at a time (#77)

.github/workflows/ci.yml

@@ -11,10 +11,8 @@ on:
     tags: '*'
   workflow_dispatch:
 concurrency:
-  # Skip intermediate builds: always.
-  # Cancel intermediate builds: only if it is a pull request build.
-  group: ${{ github.workflow }}-${{ github.ref }}
-  cancel-in-progress: ${{ startsWith(github.ref, 'refs/pull/') }}
+  group: integration-tests-global-lock
+  cancel-in-progress: false
 jobs:
   test:
     name: Julia ${{ matrix.version }} - ${{ matrix.os }} - ${{ matrix.arch }} - ${{ github.event_name }}