Only deploy an ephemeral Storybook preview for PRs that make changes to the UI package

[jessepinho]

Manual testing:

• I pushed up b598f3a (the Github workflow change) and observed that this PR did not deploy an ephemeral Storybook preview URL.
• I then pushed up 0872ecb (a temporary change to a file inside packages/ui/src) and observed that this PR did deploy an ephemeral Storybook preview URL.
• I then git reset --hard b598f3a to go back to the first commit. Note that the github-actions comment for the ephemeral Storybook URL is still there — it doesn't get auto-deleted.

Closes prax-wallet/prax#113

[changeset-bot]

⚠️ No Changeset found

Latest commit: b598f3a

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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[github-actions]

Visit the preview URL for this PR (updated for commit 0872ecb):

https://penumbra-ui-preview--pr1515-jessepinho-auto-depl-k3qiavi1.web.app

_{(expires Thu, 25 Jul 2024 20:12:38 GMT)}

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