util: Correctly use specificed RSA keysize

[dveeden]

What problem does this PR solve?

Issue Number: #26947

Problem Summary:

What is changed and how it works?

The fix in #27393 had a small bug where it fails to actually set the RSA keysize

This should fix the time it takes to run tidbTestSerialSuite.TestTLSAuto

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• morgo
• zimulala

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[dveeden]

/cc @morgo @tiancaiamao

[zimulala]

LGTM

[zimulala]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 37efebd

[ti-chi-bot]

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