Enable manual triggering of build workflow (#136)

Add workflow_dispatch event to build-plugin.yml

Author: MGraefe

.github/workflows/build-plugin.yml

@@ -6,6 +6,8 @@ on:
       - v1.**
   pull_request:
     branches: [ "master" ]
+  workflow_dispatch:
+    inputs: {}
 
 env:
   # Customize the CMake build type here (Release, Debug, RelWithDebInfo, etc.)