chore: Skip bundle size checks for Dependabot PRs

[smockle]

Follow-up to #1265

Part of https://github.com/github/primer/issues/181

Details in https://github.com/github/primer/discussions/187

This PR skips bundle size checks for Dependabot PRs (which are currently failing because of read-only tokens).

[changeset-bot]

⚠️ No Changeset found

Latest commit: 1ee138a

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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[bscofield]