[GHA] Allow rebuild using label rebuild (openhab#4739)

* This restores functionality we had before we migrated the PR builds
  to GitHub Actions.
* Implement rebuild in ci_build workflow to make PR validation pass
* Remove label rebuild at the end of the workflow

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

Author: holgerfriedrich

.github/workflows/ci-build.yml

@@ -11,10 +11,12 @@ on:
       - 'main'
     paths-ignore:
       - '.github/**/*.md'
+    types: [opened,synchronize,reopened,labeled]
   workflow_dispatch:
 
 jobs:
   build:
+    if: ${{ (github.event_name == 'pull_request' && github.event.action != 'labeled') || github.event_name == 'push' || github.event_name == 'workflow_dispatch' || github.event.label.name == 'rebuild' }}
     strategy:
       fail-fast: false
       matrix:
@@ -181,3 +183,11 @@ jobs:
           title: CheckStyle Violations
           path: '**/checkstyle-result.xml'
           mode: inline
+
+      - name: Remove Label
+        if: ${{ always() && github.event.label.name == 'rebuild' }}
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          PR: ${{ github.event.number }}
+        run: |
+          gh pr edit $PR --remove-label rebuild