cli.t: test with the right Perl executable

t/cli.t calls scripts which run with the first perl found in the
user's PATH (usually the system perl), not with the perl used for the
build, as reported here:

    https://rt.cpan.org/Ticket/Display.html?id=129944

Thanks to Slaven Rezic for spotting this and reporting it!

t/cli.t

@@ -57,7 +57,7 @@ use testutil;
 #
 # or from the top of the source tree during development.  This can be done
 # via the following, which also follows the KISS principle:
-my $STOW = 'bin/stow';
+my $STOW = "$^X bin/stow";
 
 `$STOW --help`;
 is($?, 0, "--help should return 0 exit code");