Put the chroot inside a directory that isn't group/world-accessible

Previously, the .chroot directory had permission 750 or 755 (depending
on the uid-range system feature) and was owned by root/nixbld. This
makes it possible for any nixbld user (if uid-range is disabled) or
any user (if uid-range is enabled) to inspect the contents of the
chroot of an active build and maybe interfere with it (e.g. via /tmp
in the chroot, which has 1777 permission).

To prevent this, the root is now a subdirectory of .chroot, which has
permission 700 and is owned by root/root.

(cherry picked from commit ede95b1fc1)

src/libstore/unix/build/local-derivation-goal.hh

@@ -65,6 +65,16 @@ struct LocalDerivationGoal : public DerivationGoal
      */
     bool useChroot = false;
 
+    /**
+     * The parent directory of `chrootRootDir`. It has permission 700
+     * and is owned by root to ensure other users cannot mess with
+     * `chrootRootDir`.
+     */
+    Path chrootParentDir;
+
+    /**
+     * The root of the chroot environment.
+     */
     Path chrootRootDir;
 
     /**