scripts/enum-extract.pl should not hard code perl path

This is a portability issue. The issue had already been fixed for
scripts/cstyle.pl by 2dbf1bf8296f66f24d5e404505c991bfbeec7808.
scripts/enum-extract.pl was added to the repository the following year
without this portability fix.

Michael Bishop informed me that this broke his attempt to build ZFS
2.1.6 on NixOS, since he was building manually outside of their package
manager (that usually rewrites the shebangs to NixOS' unusual paths).
NixOS puts all of the paths into $PATH, so scripts that portably rely
on env to find the interpreter still work.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14012