Adjust github actions workflow triggers (#221)

I think this might be the sauce needed to trigger:

- test workflows only on pull requests against main branch and
pushes to main branch (merges or direct pushes)
- code coverage analysis only on pushes to main branch

Closes #220

Author: lread

.github/workflows/code-coverage.yml

@@ -1,6 +1,9 @@
 name: Code Coverage
 
-on: push
+on:
+  push:
+    branches:
+    - main
 
 jobs:
   code-coverage:

.github/workflows/libs-test.yml

@@ -1,6 +1,12 @@
 name: Libs Test
 
-on: [push, pull_request]
+on:
+  push:
+    branches:
+    - main
+  pull_request:
+    branches:
+    - main
 
 jobs:
   enumerate-libs:

.github/workflows/native-image-test.yml

@@ -1,6 +1,12 @@
 name: Native Image Tests
 
-on: [push, pull_request]
+on:
+  push:
+    branches:
+    - main
+  pull_request:
+    branches:
+    - main
 
 jobs:
   build:

.github/workflows/unit-test.yml

@@ -2,8 +2,11 @@ name: Unit Tests
 
 on:
   push:
-    branches: ['main']
+    branches:
+    - main
   pull_request:
+    branches:
+    - main
 
 jobs:
   setup: