Fixed a bug where workflow trigger got reverted back to push

Author: Thomas-Boi

.github/workflows/build_icons.yml

@@ -1,5 +1,11 @@
 name: Build Icons
-on: push
+on: 
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
 jobs:
   build:
     name: Get Fonts From Icomoon