Fix build command path in GitHub Actions workflow

The build command path in the GitHub Actions workflow file was incorrect. This update removes the unnecessary backslash at the start of the path to ensure the build process runs correctly on the CI server.

.github/workflows/main.yml

@@ -31,7 +31,7 @@ jobs:
 
       # Build the solution
       - name: Build solution
-        run: msbuild "\source\Plugins.sln" /p:Configuration=Release
+        run: msbuild "source\Plugins.sln" /p:Configuration=Release
 
           # Run tests (optional)
           #- name: Run tests