Test the error message when curl is needed but not installed.

  $ make_lockdir

  $ makepkg() {
  > make_lockpkg $1 <<EOF
  > (source
  >  (fetch
  >   (url "http://0.0.0.0:8000")))
  > (version dev)
  > EOF
  > }

  $ makepkg foo

Build the package in an environment without curl.
  $ PATH=$(dirname $(which dune)) build_pkg foo
  File "dune.lock/foo.pkg", line 3, characters 7-28:
  3 |   (url "http://0.0.0.0:8000")))
             ^^^^^^^^^^^^^^^^^^^^^
  Error: The program 'curl' does not appear to be installed. Dune uses 'curl'
  to download packages. Dune requires that the 'curl' executable be located in
  one of the directories listed in the PATH variable.
  Hint: Install 'curl' with your system package manager.
  [1]
