Spurce up CI file and switch to micromamba

[mikemhenry]

• Switch to micromamba, much faster than conda
• Remove references to master branch which we don't need
• Remove extra comments
• Set codecov to no try and run on forks + schedule runs
• Update github checkout to newest version

[mikemhenry]

Now for macos it takes 3m instead of 7m to setup conda 😎

[mikemhenry]

Solves #15