t1300: fix unset of GIT_CONFIG_NOSYSTEM leaking into subsequent tests

t1300: fix unset of GIT_CONFIG_NOSYSTEM leaking into subsequent tests

In order to test whether the new GIT_CONFIG_SYSTEM environment variable
behaves as expected, we unset GIT_CONFIG_NOSYSTEM in one of our tests in
t1300. But because tests are not executed in a subshell, this unset
leaks into all subsequent tests and may thus cause them to fail in some
environments. These failures are easily reproducable with `make
prefix=/root test`.

Fix the issue by not using `sane_unset GIT_CONFIG_NOSYSTEM`, but instead
just manually add it to the environment of the two command invocations
which need it.

Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>