github: Run the CI jobs on PR

The default branch has been renamed to main. Thus, the GH workflows have
to be changed as well.

.github/workflows/integration-tests.yml

@@ -3,7 +3,7 @@ name: Coriolis Integration Tests
 on:
   workflow_dispatch:
   pull_request:
-    branches: [ "master" ]
+    branches: [ "main" ]
 
 permissions:
   contents: read

.github/workflows/unit-tests.yml

@@ -3,7 +3,7 @@ name: Coriolis Unit Tests
 on:
   workflow_dispatch:
   pull_request:
-    branches: [ "master" ]
+    branches: [ "main" ]
 
 permissions:
   contents: read