mkfs-util: set timezone to UTC when copying files into fat partition

mkfs-util: set timezone to UTC when copying files into fat partition

mcopy will set the modification time of created directories to the mtime
of the source directories but converts it to the timezone of the host.
This behavior is identical to Windows / DOS:

>  The FAT file system stores time values based on the local time of the computer.

-- https://learn.microsoft.com/en-us/windows/win32/sysinfo/file-times

To achieve reproducible builds, mcopy should be invoked with TZ=UTC.

Co-authored-by: Paul Meyer <49727155+katexochen@users.noreply.github.com>