t5000: simplify gzip prerequisite checks

In t5000, we test the built-in ".tar.gz" config for
git-archive. To make our tests portable, we check that we
have a way to both gzip and gunzip, and we respected
environment variables to point to alternate commands for
doing these operations.

However, the $GZIP variable did not actually do anything, as
changing it would not affect the baked-in value in
archive-tar.c. Moreover, setting the variable $GZIP
influences gzip itself. From the gzip man page:

  The environment variable GZIP can hold a set of default
  options for gzip. These options are interpreted first and
  can be overwritten by explicit command line parameters.

We could rename this variable, and use it to set up custom
config (or even have a Makefile knob to affect the built
binary), but it is not worth the trouble; nobody has ever
reported a problem with the baked-in default, and they can
always change it via config if they need to. Let's just drop
the variable and use "gzip" in the test (keeping the
prerequisite, of course).

While we're at it, we can drop the GUNZIP variable and
prerequisite; it uses "gzip -d", so if we have GZIP, we
will have both.

We can also use test_lazy_prereq for the gzip prerequisite,
which is simpler and behaves more consistently with the rest
of git (e.g., by making output available when the test is
run with "-v").

Noticed-by: Christian Hesse <mail@eworm.de>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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