Improve CI

Let's run the CI in a cohesive way only when master is impacted.

Author: pgrange

.github/workflows/bash_unit.yml

@@ -1,6 +1,9 @@
-name: bash_unit CI
+name: bash_unit tests
 on:
   push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
 
 jobs:
   ubuntu:

.github/workflows/pre-commit.yml

@@ -1,8 +1,10 @@
 ---
 name: pre-commit
 on:
-  pull_request:
   push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
 jobs:
   pre-commit:
     runs-on: ubuntu-latest