build on pull requests, drop per-branch push builds

A push builds in the repository it was pushed to, so a contributor's
branch builds in their fork and the base repository sees nothing. Without
a run of its own there is no check on the pull request for branch
protection to require, so trigger on pull_request as well.

Push builds narrow to master and v* tags: with pull requests covered,
building every branch push only duplicated the same commit's build across
four Windows matrix jobs. Master still builds so the "latest" pre-release
keeps updating. A branch with no pull request open builds via
workflow_dispatch.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/build.yml

@@ -3,9 +3,10 @@ name: build
 on:
   push:
     branches:
-      - '**'
+      - master
     tags:
       - 'v*'
+  pull_request:
   # allows to manually trigger a build
   workflow_dispatch:
     inputs: