feat: fix duplicate CI workflow runs on pull requests

[Copilot]

Pull requests were triggering duplicate workflow runs—once for pull_request and once for push to the PR branch. Jobs like dry-run-semantic-release and release-build-version were appearing with both (pull_request) and (push) suffixes.

Changes

Changed workflow triggers in build.yml and infersharp.yml:

# Before
on: [push, pull_request]

# After  
on:
  push:
    branches:
      - master
      - main
      - 1.x
  pull_request:

This restricts push triggers to main branches only. PRs now run once via pull_request event. Direct pushes and merged PRs trigger via push event as expected.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[bug] needs to fix CI for strange duplicated checks in PR</issue_title>
<issue_description>The following duplicated CI checks ran in this open PR: #392

[Build / build (pull_request)](https://github.com/casbin/Casbin.NET/actions/runs/18989849083/job/54240566726?pr=392)Successful in 5m
[Build / build (push)](https://github.com/casbin/Casbin.NET/actions/runs/18989848647/job/54240566485?pr=392)
Build / build (push)Successful in 5m
[Build / dry-run-semantic-release (push)](https://github.com/casbin/Casbin.NET/actions/runs/18989848647/job/54240741075?pr=392)
Build / dry-run-semantic-release (push)Successful in 23s
[Build / release-build-version (push)](https://github.com/casbin/Casbin.NET/actions/runs/18989848647/job/54240741078?pr=392)
Build / release-build-version (push)Successful in 4m
[casbin.Casbin.NET](https://github.com/casbin/Casbin.NET/pull/392/checks?check_run_id=54240796274)
casbin.Casbin.NETSuccessful in 8m — Build casbin/Casbin.NET#20251101.1 succeeded
[InferSharp / InferSharp (pull_request)](https://github.com/casbin/Casbin.NET/actions/runs/18989849067/job/54240566732?pr=392)
InferSharp / InferSharp (pull_request)Successful in 3m
[InferSharp / InferSharp (push)](https://github.com/casbin/Casbin.NET/actions/runs/18989848657/job/54240566735?pr=392)
InferSharp / InferSharp (push)Successful in 3m

I think for an open state PR, it should not run "XXX (push)" checks, right? It should run for the git push after PR merge?
Need to fix the CI.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [bug] needs to fix CI for strange duplicated checks in PR #393

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[github-actions]

🎉 This PR is included in version 2.18.0 🎉

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