playground: fix cp error when encountered readonly files

[breezewish]

What problem does this PR solve?

tiup playground cannot start due to copying Grafana files failed

What is changed and how it works?

Ignore errors and continue! Besides, -f will try to resolve read-only files by removing them first.

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:

Fix playground start error in MacOS

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• AstroProfundis

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[AstroProfundis]

/merge

[ti-chi-bot]

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

Commit hash: f63c820