nix: set 'build-use-sandbox = false' for now.

This makes nix work again; tested and verified on my system.

Close #18970

srcpkgs/nix/template

@@ -1,7 +1,7 @@
 # Template file for 'nix'
 pkgname=nix
 version=2.3.3
-revision=1
+revision=2
 build_style=gnu-configure
 # Use /nix/var as suggested by the official Manual.
 configure_args="--localstatedir=/nix/var --with-sandbox-shell=/bin/sh"
@@ -66,6 +66,7 @@ post_install() {
 	echo 'export NIX_REMOTE=daemon' > ${DESTDIR}/etc/profile.d/nix-daemon.sh
 	# Setup build users.
 	echo 'build-users-group = nixbld' > ${DESTDIR}/etc/nix/nix.conf
+	echo 'build-use-sandbox = false' >> ${DESTDIR}/etc/nix/nix.conf
 	# Install our profile.d/nix for multi-user by default.
 	vinstall ${FILESDIR}/nix.sh 644 etc/profile.d
 	# runit service