File::Spec::Unix->tmpdir: Always return an absolute path
This is generally a non-issue, however, if /tmp doesn't exist
and $ENV{TMPDIR} isn't set, ->tmpdir() used to return ".", which
broke the following pattern:
use File::Temp qw(tempdir);
use File::Spec;
my $tmpdir = tempdir(CLEANUP => 1);
chdir $tmpdir;
my $file = File::Spec->catfile($tmpdir, "foo");
open my $fh, ">", $file or die $!;
Because $tmpdir would be something like 'bfhskjf94589', and after
the chdir, the open() would've tried to open $tmpdir/$tmpdir/foo.
Note that this is only the case when not running in taint mode. If taint
is enabled, ->tmpdir will still return '.'; this mirrors the behavior of
File::Temp.