[Bug #21144] Win32: Use Windows time zone ID if TZ is not set
If the TZ environment variable is not set, the time zone names
retrieved from the system are localized for UI display and may vary
across editions and language packs for the same time zone.
Use the time zone IDs that are invariant across environments instead.
[Bug #21144] Win32: Convert the time zone name to the current locale
The Windows time zone IDs provided by Microsoft as of 24H1 are ASCII
only all, but the API itself is not impossible to set non-ASCII key
name. Prefer the current locale encoding for now until we move to
UTF-8 including environment variables and command line arguments.
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