[perl #89940] File::Spec->tmpdir and env changes

[perl #89940] File::Spec->tmpdir and env changes

If the environment changes, File::Spec->tmpdir should respect that and
recalculate the temp directory.

The only objection raised in the ticket was that File::Spec is already
accused of being too slow, so removing the cache would not be a good
idea.  So I put in a check to see whether the relevant environment
variables have changed since the last call to tmpdir.