CI: Limit backports to pixi only

.github/workflows/CI_master.yml

@@ -26,14 +26,21 @@
 
 name: FreeCAD master CI
 
-on: [workflow_dispatch, push, pull_request, merge_group]
+on:
+  workflow_dispatch: ~
+  push:
+    branches:
+      - main
+      - releases/**
+  pull_request: ~
+  merge_group: ~
+
 
 concurrency:
   group: FC-CI-${{ github.head_ref || github.run_id }}
   cancel-in-progress: true
 
 jobs:
-
   Prepare:
     uses: ./.github/workflows/sub_prepare.yml
     with:
@@ -47,18 +54,21 @@ jobs:
 
   Ubuntu:
     needs: [Prepare]
+    if: "!startsWith(github.ref, 'refs/heads/backport-')"
     uses: ./.github/workflows/sub_buildUbuntu.yml
     with:
       artifactBasename: Ubuntu-${{ github.run_id }}
 
   Windows:
     needs: [Prepare]
+    if: "!startsWith(github.ref, 'refs/heads/backport-')"
     uses: ./.github/workflows/sub_buildWindows.yml
     with:
       artifactBasename: Windows-${{ github.run_id }}
 
   Lint:
     needs: [Prepare]
+    if: "!startsWith(github.ref, 'refs/heads/backport-')"
     uses: ./.github/workflows/sub_lint.yml
     with:
       artifactBasename: Lint-${{ github.run_id }}