chore: avoid duplicate CI runs on branch push + pull_request

push and pull_request both fired for commits on branches with an open
PR in this repo, running each workflow twice. Restrict the push
trigger to main so branch work is covered by pull_request alone.

Author: madogiwa0124

.github/workflows/js.yml

@@ -1,6 +1,9 @@
 name: JS Build & Test
 
-on: [push, pull_request]
+on:
+  push:
+    branches: [main]
+  pull_request:
 
 jobs:
   build:

.github/workflows/rubocop.yml

@@ -1,6 +1,9 @@
 name: Rubocop
 
-on: [push, pull_request]
+on:
+  push:
+    branches: [main]
+  pull_request:
 
 jobs:
   build:

.github/workflows/ruby.yml

@@ -1,6 +1,9 @@
 name: build
 
-on: [push, pull_request]
+on:
+  push:
+    branches: [main]
+  pull_request:
 
 jobs:
   build: