ci: simplify CI EV install, use apt-get, remove conda (#600)
We don't actually need conda, at least not anymore:
- sox and ffmpeg can be installed with apt-get
- previously optimized pip installs for pyworld and pycountry can simply be
cached by the standard pip caching mechanism, instead of using custom
optimizations
The result is a more standard CI workflow, and a tiny bit faster to boot.
