[main](backport #6651) use centralized version qualifier

.buildkite/pipeline.elastic-agent-binary-dra.yml

@@ -47,8 +47,9 @@ steps:
     if: build.branch =~ /^[0-9]+\.[0-9x]+\$/ || build.env("RUN_STAGING") == "true" || build.env('VERSION_QUALIFIER') != null
     steps:
     - label: ":package: Build Elastic-Agent Core staging"
-      commands:
-        - .buildkite/scripts/steps/build-agent-core.sh
+      commands: |
+        source .buildkite/scripts/version_qualifier.sh
+        .buildkite/scripts/steps/build-agent-core.sh
       key: "build-dra-staging"
       artifact_paths:
         - "build/distributions/**/*"
@@ -62,6 +63,7 @@ steps:
 
     - label: ":hammer: DRA Publish Elastic-Agent Core staging"
       command: |
+        source .buildkite/scripts/version_qualifier.sh
         echo "+++ Restoring Artifacts"
         buildkite-agent artifact download "build/**/*" .
         echo "+++ Changing permissions for the release manager"

.buildkite/scripts/version_qualifier.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+
+# An opinionated approach to managing the Elastic Qualifier for the DRA in a Google Bucket
+# instead of using a Buildkite env variable.
+
+if [[ -n "$VERSION_QUALIFIER" ]]; then
+  echo "~~~ VERSION_QUALIFIER externally set to [$VERSION_QUALIFIER]"
+  return 0
+fi
+
+# DRA_BRANCH can be used for manually testing packaging with PRs
+# e.g. define `DRA_BRANCH="main"` under Options/Environment Variables in the Buildkite UI after clicking new Build
+BRANCH="${DRA_BRANCH:="${BUILDKITE_BRANCH:=""}"}"
+
+qualifier=""
+URL="https://storage.googleapis.com/dra-qualifier/${BRANCH}"
+if curl -sf -o /dev/null "$URL" ; then
+  qualifier=$(curl -s "$URL")
+fi
+
+export VERSION_QUALIFIER="$qualifier"
+echo "~~~ VERSION_QUALIFIER set to [$VERSION_QUALIFIER]"