build.cc: fix bind-mount of /dev/{pts,ptmx} fallback

Don't bind-mount these to themselves,
mount them into the chroot directory.

Fixes pty issues when using sandbox on CentOS 7.4.
(build of perlPackages.IOTty fails before this change)

src/libstore/build.cc

@@ -2699,8 +2699,8 @@ void DerivationGoal::runChild()
                 } else {
                     if (errno != EINVAL)
                         throw SysError("mounting /dev/pts");
-                    doBind("/dev/pts", "/dev/pts");
-                    doBind("/dev/ptmx", "/dev/ptmx");
+                    doBind("/dev/pts", chrootRootDir + "/dev/pts");
+                    doBind("/dev/ptmx", chrootRootDir + "/dev/ptmx");
                 }
             }