appveyor: install zlib and minor clean ups

appveyor: install zlib and minor clean ups

- Install the appropriate mingw64 version of zlib so fio compile finds
  and uses it.
- Always use Cygwin64. Appveyor always provides a 64 bit environment and
  Cygwin64 comes with a 32 bit cross-compiler and 32 bit versions of
  libraries.
- Reduce the clone depth to 1 since we aren't making use of the git
  history and it can be a touch faster.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>