[build-in-chroot.sh] Changelog:

* Remove chroot directory created by "mktemp -d" before running
  mkarchroot. This is necessary due to the removal of the '-f' option
  for mkarchroot. This could potentially cause problems when building
  two packages in the same directory, but that's not a normal use case.
* mkarchroot no longer passes the argument to '-r' to eval. This means
  that the following command will not work:

    mkarchroot -r "foo && bar" ...

  Instead, the script will now write the commands to a script in the
  chroot and run:

    mkarchroot -r "/stage#" ...