go-misc: attempting new appveyor 2.0 config

Current appveyor setup seems broken. Some of that may be my fault, as I
am not using the provided source clone, and instead am just using go get
as run by a windows machine.

The new configuration will hopefully allow the pre-cloned code to be used
directly, and as long as no go gets are needed, the tests should run
properly.

Also taking advantage of build matrix to test windows/386 and windows/amd64
at the same time.