ci: tzdata update workflow

[mitsu-yuki]

Hello,

This pull request introduces a workflow that checks daily for updates to tzdata.tar.gz at 23:00+00:00 (UTC), which is 08:00 JST.
If changes are detected, the source code will be updated, and go run scripts/tzgen/main.go will be executed.
The regenerated tz/ directory, along with other changes, will be committed to the main branch.

Example of operation:
When an update occurs:

• Commit: Update tzdata to 2025c · mitsu-yuki/synchro@cc40d80
• Workflow: tzdata update · mitsu-yuki/synchro@8ca9095

Changes Made

• Refactored the tzdata URL to be constructed from tzVersion.
• Added a workflow to automatically check and update tzdata daily to its latest version.

Warning

• This workflow has write permissions.
• Commits will be made directly to the main branch, bypassing the creation of pull requests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (3f7201f) to head (8de0073).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main       #41    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           16        16            
  Lines         2672      2215   -457     
==========================================
- Hits          2672      2215   -457     

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