Remove usage of conda in Windows CI

[letmaik]

Follow-up from #150 where reliance on Conda was reduced to a minimum already.

Instead, use python from PATH together with virtualenv. A downside is that it is locally slightly more work to produce wheels for multiple Python versions, but the main use case of the build scripts is for CI anyway, and there we can rely on the environment.

Removing reliance on Conda paves the way for more automated tooling like cibuildwheel.

[letmaik]

This was done in #150. Conda is still available as alternative build option, mostly for local builds.