chore: add label build-image-on-pr to trigger a build

[tabVersion]

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What's changed and what's your intention?

as title, a new label to trigger a workflow to build a new image on the current branch, and labeled as the branch name.

note that the image tag is sanitized: feature/new-thing becomes feature-new-thing

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added test labels as necessary. See details.
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Release note

If this PR includes changes that directly affect users or other significant modifications relevant to the community, kindly draft a release note to provide a concise summary of these changes. Please prioritize highlighting the impact these changes will have on users.

[kwannoel]

You can also add the github label to this PR, to test it works.

[BugenZhao]

Some concerns:

• It seems that we build the image directly based on the commits in the branch, instead of merging it into the main branch first. I'm afraid there can be confusion when different base commits are used.
• What if the workflow is triggered multiple times on the same branch, given that we only encode the branch name into the image tag.