daemon: Rename 'NIX_STATE_DIR' and 'NIX_DB_DIR' environment variables.

daemon: Rename 'NIX_STATE_DIR' and 'NIX_DB_DIR' environment variables.

Fixes <https://bugs.gnu.org/22459>.
Reported by Jeff Mickey <j@codemac.net>.

* guix/config.scm.in (%state-directory): Change NIX_STATE_DIR to
GUIX_STATE_DIRECTORY.
(%store-database-directory): Change NIX_DB_DIR to
GUIX_DATABASE_DIRECTORY.
* nix/libstore/globals.cc (Settings::processEnvironment): Likewise.
* guix/self.scm (make-config.scm): Likewise.
* build-aux/build-self.scm (make-config.scm): Likewise.
* build-aux/test-env.in: Likewise.
* tests/derivations.scm ("derivation #:leaked-env-vars"): Likewise.
* tests/guix-build.sh (GUIX_DAEMON_SOCKET): Likewise.
* tests/guix-daemon.sh (socket): Likewise.