build: don't run test-asan workflow on non-master pushes

PR-URL: #34509
Refs: #34464
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>

.github/workflows/test-asan.yml

@@ -2,6 +2,8 @@ name: test-asan
 
 on:
   push:
+    branches:
+    - master
     paths-ignore:
     - 'doc/**'
   pull_request: