git-credential-netrc: use in-tree Git.pm for tests

The netrc test.pl script calls git-credential-netrc which imports the
Git module.  Pass GITPERLLIB to git-credential-netrc via PERL5LIB to
ensure the in-tree Git module is used for testing.

Signed-off-by: Luis Marsano <luis.marsano@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>