Fix no need mirror be set --version and --help

[u5surf]

What problem does this PR solve?

close #2411

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release notes:

NONE

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[ti-chi-bot]

Welcome @u5surf! It looks like this is your first PR to pingcap/tiup 🎉

[pingcap-cla-assistant]

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