Prevent concurrent builds on branches (#406)

Prevent multiple builds at the same time from the same branch by cancelling the earlier builds. The default branch (e.g. master) is exempted, that is, builds from the default branch may run concurrently.

.github/workflows/ci.yml

@@ -15,6 +15,9 @@ env:
   pull_request:
     branches-ignore:
     - gh-pages
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref == format('refs/heads/{0}', github.event.repository.default_branch) && github.run_id || github.ref }}
+  cancel-in-progress: true
 jobs:
   build:
     name: Build

zio-sbt-githubactions/src/main/scala/zio/sbt/githubactions/model.scala

@@ -316,6 +316,12 @@ object Workflow {
                          .map(_.toKeyValuePair): _*
                      )
                    }),
+          "concurrency" := Json.obj(
+            "group" := Json.fromString(
+              "${{ github.workflow }}-${{ github.ref == format('refs/heads/{0}', github.event.repository.default_branch) && github.run_id || github.ref }}"
+            ),
+            "cancel-in-progress" := true
+          ),
           "jobs" := Json.obj(wf.jobs.map(job => job.id := job): _*)
         )
 }