Add missing output to build job

Author: PhilippeChab

.github/workflows/build.yml

@@ -10,6 +10,8 @@ jobs:
   build:
     name: Build
     runs-on: ubuntu-latest
+    outputs:
+      vsixPath: ${{ steps.packageExtension.outputs.vsixPath }}
     steps:
       - name: Checkout
         uses: actions/checkout@v2