chore: run workflows only on PRs to main and main itself

Author: mwwoda

.github/workflows/build-main.yml

@@ -2,6 +2,8 @@ name: build-main
 on:
   pull_request:
     types: [ opened, synchronize ]
+    branches:
+      - main
   push:
     branches:
       - main

.github/workflows/semantic-pr.yml

@@ -6,6 +6,8 @@ on:
       - opened
       - edited
       - synchronize
+    branches:
+      - main
 
 jobs:
   main: