Make file2lastrev timezone consistent with git log

Using the same timezone for all commits is convenient when just looking
dates in RUBY_DESCRIPTION, but usually we also check `git log` when
we're interested in the order of commits.
`git log` shows times in committer's timezone and forcing RUBY_RELEASE_DATE
to JST makes it harder to find a corresponding commit from `git log`.

Because this label is only used in development, I believe there's no
strict requirement to use traditional timezone for release here.

Also when building Ruby after committing from a non-JST timezone, I'd be
surprised to see a strange time (in a different timezone) for my very new
commit in `ruby -v`.

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