.appveyor.yml: enable Ubuntu builds [ci skip]
.appveyor.yml: enable Ubuntu builds [ci skip]

- work directory left to its default to let it work on Ubuntu
- Ubuntu provided by AppVeyor misses 'wine' packages so
  building openssl and curl will fail when trying to run the
  target .exe files. This can be solved by using Docker, the
  same way it is done on Travis CI. (Though this would make
  the two builds identical, so running them both makes little
  sense). Ubuntu builds will be interesting when Bionic
  (or other recent/rolling) image gets deployed.

vszakats/harbour-deps,vszakats/harbour-deps