Don't install Nix

[asymmetric]

See https://github.com/dapphub/dapptools/pull/377.

Also switches to GitHub actions for CI.

NOTE:

• only runs on GNU/Linux, because GitHub rate-limits the IP the macOS builders are on
• Don't know how this fails when nix is not installed, since the shebang uses nix-env
• The current latest dapptools release (ethsign/0.15.0) is getting compiled on both CI and my machine. Not sure why this is, hopefully it goes away with the next release.

Closes #15. Closes #16. Closes dapphub/dapptools#375.

[asymmetric]

The macOS failure seems to be a known issue. I'm thinking the thing to do here is to only run the CI on Linux - we'll see if GitHub fixes the issue on their end.