daemon: Run 'guix offload' directly.

* nix/scripts/offload.in: Remove.
* nix/local.mk (nodist_pkglibexec_SCRIPTS) [BUILD_DAEMON_OFFLOAD]:
Remove 'scripts/offload'.
* config-daemon.ac: Don't output 'nix/scripts/offload'.
* build-aux/pre-inst-env.in: Don't set 'NIX_BUILD_HOOK'.
* nix/libstore/build.cc (HookInstance::HookInstance): Run 'guix
offload'.
(DerivationGoal::tryBuildHook): Remove reference to 'NIX_BUILD_HOOK'.
* nix/nix-daemon/guix-daemon.cc (main) [HAVE_DAEMON_OFFLOAD_HOOK]: Don't
set 'NIX_BUILD_HOOK'.
* nix/nix-daemon/nix-daemon.cc (performOp) [!HAVE_DAEMON_OFFLOAD_HOOK]:
Leave 'settings.useBuildHook' unchanged.