Clean after the source generations.

To make sure the build step builds as it would for a first build.

.github/workflows/ci.yml

@@ -108,6 +108,7 @@ jobs:
       - name: Generate godot-cpp sources only
         run: |
           scons platform=${{ matrix.platform }} build_library=no ${{ matrix.flags }}
+          scons -c
 
       - name: Build godot-cpp (debug)
         run: |