Concurrency for build job

.github/workflows/haskell.yml

@@ -25,6 +25,27 @@ jobs:
       # current ref from: 27.02.2022
       SECP256K1_REF: ac83be33d0956faf6b7f61a60ab524ef7d6a473a
 
+    # When pushing branches (and/or updating PRs), we do want to cancel previous
+    # build runs. We assume they are stale now; and do not want to spend CI time and
+    # resources on continuing to continue those runs. This is what the concurrency.group
+    # value lets us express. When using merge queues, we now have to consider
+    # - runs triggers by commits per pull-request
+    #   we want to cancel any previous run. So they should all get the same group (per PR)
+    # - runs refs/heads/gh-readonly-queue/<target branch name> (they should all get their
+    #   unique git ref, we don't want to cancel any of the ones in the queue)
+    # - if it's neither, we fall back to the run_id (this is a unique number for each
+    #   workflow run; it does not change if you "rerun" a job)
+    concurrency:
+      group: >
+        ${{ github.event.type }}
+        ${{ github.workflow }}
+        ${{ github.job }}
+        ${{ matrix.ghc }}
+        ${{ matrix.cabal }}
+        ${{ matrix.os }}
+        ${{ (startsWith(github.ref, 'refs/heads/gh-readonly-queue/') && github.ref) || github.event.pull_request.number || github.run_id }}
+      cancel-in-progress: true
+
     steps:
     - name: Install Haskell
       uses: input-output-hk/setup-haskell@v1