github_actions: run tests only on labels

When a github PR is made, the unit tests will start only after a
maintainer labels the PR with 'ready-for-unit-testing' label.

Change-Id: I672843926f68c232907d603d91064422e414970e

Author: ader1990

.github/workflows/unit_tests.yml

@@ -1,9 +1,13 @@
 name: Cloudbase-Init unit tests
 
-on: [push, pull_request]
+on:
+  push:
+  pull_request:
+    types: [ labeled ]
 
 jobs:
   build:
+    if: ${{ github.event_name == 'push' || contains(github.event.pull_request.labels.*.name, 'ready-for-unit-testing') }}
     runs-on: ubuntu-22.04
     strategy:
       matrix: