feat: add Github Actions environment variable to isPullRequest method

[kuidaoring]

add GithubActions environment variable to isPullRequest method.
GITHUB_BASE_REF environment variable is set in pull request build.
see: https://docs.github.com/en/actions/learn-github-actions/environment-variables#default-environment-variables

[changeset-bot]

🦋 Changeset detected

Latest commit: 849eb89

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 10 packages

Name	Type
builder-util	Patch
app-builder-lib	Patch
dmg-builder	Patch
electron-builder-squirrel-windows	Patch
electron-builder	Patch
electron-publish	Patch
electron-forge-maker-appimage	Patch
electron-forge-maker-nsis-web	Patch
electron-forge-maker-nsis	Patch
electron-forge-maker-snap	Patch

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