Add GitHub action to build for Raspberry Pi aarch64

[ideoforms]

Should be possible with this action that uses qemu to emulate aarch64 in userland:
https://github.com/pguyot/arm-runner-action

[psytron]

This would be awesome! :)

Right now builders and enthusiasts who are on Raspberry Pi get:

ERROR: No matching distribution found for signalflow 

[ideoforms]

I'm keen to make this happen, but in the meantime, it's also super easy to build from source! It takes a while (10mins+) on Raspberry Pi, but these instructions should work:

https://github.com/ideoforms/signalflow/blob/master/CONTRIBUTING.md#building-from-source

Let me know how you get on.