Add concurrency control and restrict publishing

- Introduced concurrency to cancel in-progress runs on the same ref.
- Limited workflow execution to the `main` branch.

Author: ndonkoHenri

.github/workflows/build-python-version.yml

@@ -188,6 +188,7 @@ jobs:
       - build-android
       - build-linux
       - build-windows
+    if: github.ref == 'refs/heads/main'
     steps:
       - name: Derive short Python version
         shell: bash

.github/workflows/build-python.yml

@@ -2,10 +2,14 @@ name: Build Python Packages
 
 on:
   push:
-    branches:
-      - '**'
+  pull_request:
   workflow_dispatch:
 
+# Cancel in-flight runs when a newer commit lands on the same ref.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   build-matrix:
     name: Build Python ${{ matrix.python_version }}