CI: Add manual trigger for 'workflow_run' builds (#7865)

.github/workflows/build.yml

@@ -8,6 +8,7 @@ on:
     # "scheduled" workflow, while maintaining ability to perform local CI builds.
     workflows:
       - scheduled
+      - manual
     branches:
       - master
       - docking

.github/workflows/manual.yml

@@ -0,0 +1,12 @@
+#
+# This is a dummy workflow used to trigger full builds manually.
+#
+name: manual
+
+on: workflow_dispatch
+
+jobs:
+  manual:
+    runs-on: ubuntu-latest
+    steps:
+      - run: exit 0