ci: skip running preview workflow in forks

[bluwy]

In my fork branch I got an error when running the preview workflow. A repo check should help prevent it and potentially mistakenly publishing it if someone has pkg.pr.new app setup.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 8c0293c

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[pkg-pr-new]

Open in Stackblitz • @example/basic • @example/changesets

npm i https://pkg.pr.new/bombshell-dev/clack/@clack/core@254

npm i https://pkg.pr.new/bombshell-dev/clack/@clack/prompts@254

commit: 8c0293c

[dreyfus92]

nice catch 🫶🏻