Skip pull-request-build on master push (#6934)

The runs pushes to master always fail because PR_NUMBER is absent, so
the script invocation fails:

[2026-05-01T17:28:57.403Z] --pr-number value is required!
Error: Process completed with exit code 1.

Running this on master push is redunant anyway because PR's go through
the merge queue first, where this script *does* run correctly.

Author: dagnir

.github/workflows/pull-request-build.yml

@@ -2,9 +2,6 @@ name: Build SDK
 on:
   merge_group:
   pull_request:
-  push:
-    branches:
-      - master
     paths-ignore:
       - '**.md'
       - '.all-contributorsrc'