chore: add support for feature branch build

[purelind]

Add support on multibranch build on feature branch of tidb & tiflash.

[ti-chi-bot]

I have already done a preliminary review for you, and I hope to help you do a better job.

This pull request adds support for feature branch builds for tidb & tiflash. The key changes are:

• If the branch starts with feature/, the script will use the master branch to build plugins.
• In build_tiflash_multi_branch.groovy, image_tag is set to ${env.BRANCH_NAME} with forward slashes replaced by dashes.

Overall, the changes look good and the code appears to be working as expected. However, there are a few potential problems:

• It might be better to explicitly check if env.TAG_NAME is null instead of relying on a ternary operator.
• It might be good to add some error handling in case the plugin branch cannot be checked out.
• In build_tiflash_multi_branch.groovy, if the branch name contains special characters, it may cause issues when using it as the image tag.

To fix these potential problems:

• Change the branch variable to explicitly check if env.TAG_NAME is null.
• Add catch clauses to handle any exceptions when checking out the plugin branch.
• Sanitize the branch name before using it as the image tag.

Overall, the changes look good and the potential problems are minor. I would approve this pull request.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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