build-fhs-userenv: move /tmp handling to bash part

pkgs/build-support/build-fhs-userenv/chroot-user.rb

@@ -14,10 +14,6 @@ mounts = [ ['/nix/store', nil],
            ['/root', nil],
          ]
 
-# Create directories
-mkdirs = ['tmp',
-         ]
-
 # Propagate environment variables
 envvars = [ 'TERM',
             'DISPLAY',
@@ -100,9 +96,6 @@ if $cpid == 0
   write_file '/proc/self/uid_map', "#{uid} #{uid} 1"
   write_file '/proc/self/gid_map', "#{gid} #{gid} 1"
 
-  # Do mkdirs
-  mkdirs.each { |x| FileUtils.mkdir_p "#{root}/#{x}" }
-
   # Do rbind mounts.
   mounts.each do |x|
     to = "#{root}/#{x[1]}"

pkgs/build-support/build-fhs-userenv/default.nix

@@ -10,6 +10,9 @@ let
   '';
 
   init = writeText "init" ''
+           # Make /tmp directory
+           mkdir -m 1777 /tmp
+
            # Expose sockets in /tmp
            for i in /host-tmp/.*-unix; do
              ln -s "$i" "/tmp/$(basename "$i")"