appveyor: attempt to cache the _build/opam directory
appveyor: attempt to cache the _build/opam directory

The cache will be invalidated if the repo/win32 directory containing
the package definitions is modified.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit