mount tmpfs on /tmp

too much stuff doesn't work without it and it's not
all worth patching

modules/base.nix

@@ -134,6 +134,7 @@ in {
       proc = dir {};
       run = dir {};
       sys = dir {};
+      tmp = dir {};
     };
   };
 }

modules/s6/scripts/rc.init

@@ -17,6 +17,7 @@ shift
 
 mount -t proc none /proc
 mount -t sysfs none /sys
+mount -t tmpfs none /tmp
 # s6-linux-init mounts /dev before this script is called
 mkdir /dev/pts
 mount -t devpts none /dev/pts