repository/progress: Format progress

[dveeden]

What problem does this PR solve?

This fixes #451

Instead of 1.23 MiB p/s it now shows 1.23 MiB/s.

What is changed and how it works?

This sets a format string for the speed element of the ProgressBar.

Check List

Tests

• Manual test (add detailed scripts or steps below)

Run tiup mirror clone ... and inspect output.

Related changes

• Need to cherry-pick to the release branch

Release notes:

NONE

[lonng]

lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lonng

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[lonng]

/merge

[ti-chi-bot]

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

Commit hash: 32699ab