In Nix chroots, provide plain bash, not bash-interactive

This way we don't get ncurses in the chroot. Also, use a bind-mount of
/bin/sh to bash, rather than the entire /bin from the host filesystem.

nixos/modules/services/misc/nix-daemon.nix

@@ -22,14 +22,11 @@ let
 
   nixConf =
     let
-      # Tricky: if we're using a chroot for builds, then we need
-      # /bin/sh in the chroot (our own compromise to purity).
-      # However, since /bin/sh is a symlink to some path in the
-      # Nix store, which furthermore has runtime dependencies on
-      # other paths in the store, we need the closure of /bin/sh
-      # in `build-chroot-dirs' - otherwise any builder that uses
-      # /bin/sh won't work.
-      binshDeps = pkgs.writeReferencesToFile config.system.build.binsh;
+      # If we're using a chroot for builds, then provide /bin/sh in
+      # the chroot as a bind-mount to bash. This means we also need to
+      # include the entire closure of bash.
+      sh = pkgs.stdenv.shell;
+      binshDeps = pkgs.writeReferencesToFile sh;
     in
       pkgs.runCommand "nix.conf" {extraOptions = cfg.extraOptions; } ''
         extraPaths=$(for i in $(cat ${binshDeps}); do if test -d $i; then echo $i; fi; done)
@@ -40,7 +37,7 @@ let
         build-users-group = nixbld
         build-max-jobs = ${toString (cfg.maxJobs)}
         build-use-chroot = ${if cfg.useChroot then "true" else "false"}
-        build-chroot-dirs = ${toString cfg.chrootDirs} $(echo $extraPaths)
+        build-chroot-dirs = ${toString cfg.chrootDirs} /bin/sh=${sh} $(echo $extraPaths)
         binary-caches = ${toString cfg.binaryCaches}
         trusted-binary-caches = ${toString cfg.trustedBinaryCaches}
         $extraOptions
@@ -253,8 +250,6 @@ in
 
   config = {
 
-    nix.chrootDirs = [ "/bin" ];
-
     environment.etc."nix/nix.conf".source = nixConf;
 
     # List of machines for distributed Nix builds in the format