chore: increate timeout for tiflash ut

[purelind]

In some PRs with many changes, the tiflash binary compilation time and test execution time may exceed 60 minutes, so increase the timeout limit here.

[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 increases the timeout limit for the unit tests in the tiflash pipeline from 60 minutes to 90 minutes to account for cases where the build and test execution time exceeds 60 minutes.

There are no potential problems identified in this pull request.

My suggestion would be to ensure that the timeout limit is increased proportionally to the expected build and test execution time to prevent unnecessarily long timeouts. Also, it would be good to monitor the build and test execution time regularly to ensure that the timeout limit is appropriate.

[ti-chi-bot]

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