fix(tiflash): correct build ccache path

[purelind]

correct build ccache path.

[ti-chi-bot]

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

Based on the pull request title and description, it seems that the change is related to fixing the build ccache path. The actual change is in the merged_build.groovy file, where the cp command has been updated to add .tar to the end of the file name.

There are no potential problems seen in this change, as it seems to be a straightforward fix that should not cause any issues.

One suggestion for improvement is to add more context in the pull request description, such as why this change was necessary and how it will impact the overall pipeline.

Another suggestion is to ensure that the new file name is consistent with the naming conventions used in the rest of the pipeline, to avoid any confusion or errors.

[ti-chi-bot]

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