Branch specific concurrency gates (#39298)

PR #39293 introduced one concurrency queue per staging/snapshot but
this slows down unnecessarily concurrent DRA builds for main and other
release branches.

This commit makes the concurrency gates (additionally) specific per branch.

(cherry picked from commit 40c68cf)

.buildkite/packaging.pipeline.yml

@@ -19,14 +19,14 @@ steps:
   - name: Start of concurrency group for DRA Snapshot
     if: build.branch =~ /^\d+\.\d+$$/ || build.branch == 'main' || build.env('RUN_SNAPSHOT') == "true"
     command: echo "--> Start of concurrency gate dra-snapshot"
-    concurrency_group: "dra-gate-snapshot"
+    concurrency_group: "dra-gate-snapshot-$BUILDKITE_BRANCH"
     concurrency: 1
     key: start-gate-snapshot
 
   - name: Start of concurrency group for DRA Staging
     if: build.branch =~ /^\d+\.\d+$$/
     command: echo "--> Start of concurrency gate dra-staging"
-    concurrency_group: "dra-gate-staging"
+    concurrency_group: "dra-gate-staging-$BUILDKITE_BRANCH"
     concurrency: 1
     key: start-gate-staging
 
@@ -265,12 +265,12 @@ steps:
 
   - command: echo "End of concurrency gate dra-snapshot <--"
     if: build.branch =~ /^\d+\.\d+$$/ || build.branch == 'main' || build.env('RUN_SNAPSHOT') == "true"
-    concurrency_group: "dra-gate-snapshot"
+    concurrency_group: "dra-gate-snapshot-$BUILDKITE_BRANCH"
     concurrency: 1
     key: end-gate-snapshot
 
   - command: echo "End of concurrency gate dra-staging <--"
     if: build.branch =~ /^\d+\.\d+$$/
-    concurrency_group: "dra-gate-staging"
+    concurrency_group: "dra-gate-staging-$BUILDKITE_BRANCH"
     concurrency: 1
     key: end-gate-staging