daemon: Rename 'NIX_CONF_DIR' to 'GUIX_CONFIGURATION_DIRECTORY'.

Partly fixes <http://bugs.gnu.org/22459

>.
Reported by Jeff Mickey <j@codemac.net> and David Craven <david@craven.ch>.

* nix/libstore/globals.cc (Settings::processEnvironment()): Change
'NIX_CONF_DIR' to 'GUIX_CONFIGURATION_DIRECTORY'.
* nix/local.mk (libstore_a_CPPFLAGS): Likewise.
* guix/config.scm.in (%config-directory): Likewise.
* build-aux/test-env.in: Likewise.
* gnu/packages/patches/hydra-automake-1.15.patch: Likewise.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>