web build: workflow triggered by all branches

.github/workflows/build_web.yml

@@ -1,11 +1,9 @@
 name: Web Build (Emscripten)
-on:
-  push:
-    branches:
-      wasm
+on: [push, pull_request]
 
 jobs:
-  Push:
+  Build:
+    if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name != github.repository
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v1