fix: auto-bootstrap root.json and create bin dir on first run

[yahonda]

What problem does this PR solve?

When tiup is installed via mise (https://mise.jdx.dev, a polyglot tool version manager similar to asdf) with:

mise install github:pingcap/tiup
mise use -g github:pingcap/tiup

the install.sh script is not executed, which means:

1. ~/.tiup/bin/ directory is not created
2. root.json is not downloaded from the mirror

This leaves the installation incomplete and requires the user to manually run 'tiup mirror set' before any tiup command works.

What is changed and how it works?

This PR fixes both issues:

• ResetMirror now ensures the bin directory exists before writing root.json
• InitEnv automatically bootstraps root.json from the default mirror if it's missing, so no manual 'tiup mirror set' is needed

In the future, we would like to register tiup in the mise registry so that users can simply run:

mise install tiup
mise use tiup

This fix is a prerequisite for that, as it removes the need for any post-install initialization steps.

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

[ti-chi-bot]

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 15.38462% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.29%. Comparing base (9bdae04) to head (86ccffc).

Files with missing lines	Patch %	Lines
pkg/environment/env.go	18.18%	8 Missing and 1 partial ⚠️
pkg/localdata/profile.go	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2709      +/-   ##
==========================================
- Coverage   42.72%   42.29%   -0.43%     
==========================================
  Files         424      424              
  Lines       49744    47155    -2589     
==========================================
- Hits        21253    19943    -1310     
+ Misses      25797    24512    -1285     
- Partials     2694     2700       +6     

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