Add m1/apple silicon to testing matrix

[MetricMike]

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.