Fix generation of the ppx key for jbuild directories (#860)
Fix generation of the ppx key for jbuild directories (#860)

Signed-off-by: Jeremie Dimino <21ffd895359415ee7287ceeecbfa775472db1289@janestreet.com>
dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,dra27/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder