t/lib-httpd: pass through GIT_CONFIG_NOSYSTEM env

t/lib-httpd: pass through GIT_CONFIG_NOSYSTEM env

We set GIT_CONFIG_NOSYSTEM in our test scripts so that we do
not accidentally read /etc/gitconfig and have it influence
the outcome of the tests. But when running smart-http tests,
Apache will clean the environment, including this variable,
and the "server" side of our http operations will read it.

You can see this breakage by doing something like:

  make
  ./git config --system http.getanyfile false
  make test

which will cause t5561 to fail when it tests the
fallback-to-dumb operation.

We can fix this by instructing Apache to pass through the
variable. Unlike with other variables (e.g., 89c57ab3's
GIT_TRACE), we don't need to set a dummy value to prevent
warnings from Apache. test-lib.sh already makes sure that
GIT_CONFIG_NOSYSTEM is set and exported.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>