fix(dracut.sh): harden dracut against GZIP environment variable

fix(dracut.sh): harden dracut against GZIP environment variable

When a GZIP environment variable is set, this leads to various breakage:

In case 'pigz' is installed and GZIP is defined as a path, e.g.
/usr/local/bin/gzip, then dracut will fail with the following message:

   "
   pigz: abort: cannot provide files in GZIP environment variable
   "

In case 'pigz' isn't installed and regular 'gzip' is used and GZIP is
defined as a path, e.g. /usr/local/bin/gzip, then the path will be
zipped and dracut will fail for no obvious reason.  Trying again, dracut
will then fail with following message:

   "
   gzip: /usr/local/bin/gzip.gz already exists;	not overwritten
   "

In any case, GZIP environment should be unset to avoid breakage or
unwanted behaviour. This variable is anyway obsolescent, from gzip(1)
manpage.

Signed-off-by: Renaud Métrich <rmetrich@redhat.com>