Improve conditional GitHub actions

.github/workflows/build.yml

@@ -1,5 +1,11 @@
 name: Build
-on: [push, pull_request]
+on:
+  push:
+    paths:
+      - "**.swift"
+  pull_request:
+    branches:
+      - main
 jobs:
   build:
     name: Build

.github/workflows/linting.yml

@@ -1,5 +1,11 @@
 name: Linting
-on: [push, pull_request]
+on:
+  push:
+    paths:
+      - "**.swift"
+  pull_request:
+    branches:
+      - main
 jobs:
   linting:
     name: Linting

.github/workflows/unit_tests.yml

@@ -1,5 +1,11 @@
 name: Unit Tests
-on: [push, pull_request]
+on:
+  push:
+    paths:
+      - "**.swift"
+  pull_request:
+    branches:
+      - main
 jobs:
   unit_tests:
     name: Unit Tests