Add m1/apple silicon to testing matrix

**Is your feature request related to a problem? Please describe.**

We're not testing on M1s, which led to a lot of frustration and new issues during the first half of this year.

**Describe the solution you'd like**

GitHub just released M1 runners. 
https://github.com/github/roadmap/issues/528

We should add those to the os matrix in actions.
