[GHA] Allow rebuild using label rebuild (#18596)

* This restores functionality we had before we migrated the PR builds
  to GitHub Actions.

Signed-off-by: Holger Friedrich <mail@holger-friedrich.de>

.github/workflows/ci-build.yml

@@ -12,6 +12,7 @@ on:
     paths-ignore:
       - '.github/**/*.md'
   workflow_dispatch:
+  workflow_call:
 
 jobs:
   build:

.github/workflows/rebuild.yml

@@ -0,0 +1,12 @@
+# This workflow triggers the ci_build workflow when a pull request
+# is labeled with 'rebuild'.
+name: Trigger ci_build on label
+
+on:
+  pull_request:
+    types: [labeled]
+
+jobs:
+  trigger-build:
+    if: ${{ github.event.label.name == 'rebuild' }}
+    uses: ./.github/workflows/ci-build.yml