cvsimport: work around perl tzset issue

cvsimport: work around perl tzset issue

On many platforms, the first invocation of localtime_r will
check $TZ in the environment, but subsequent invocations
will use a cached value. That means that setting $ENV{TZ} in
the middle of the program may or may not have an effect on
later calls to localtime.  Perl 5.10.0 and later handles
this automatically for us, but we try to remain portable
back to 5.8. Work around it by calling tzset ourselves.