Add arm64 builds for macOS, and build them from GH Actions

[sergiou87]

Trying to get https://github.com/desktop/desktop builds for macOS arm64, we need to build git in https://github.com/desktop/dugite-native for arm64, and in order to do that we need smimesign built for arm64 😄

I hope this is enough to make it happen 🤞

Spoiler: it doesn't yet, I'll work on this 😂

After some work it seems Travis doesn't want to help me get those builds, so I directly migrated the macOS CI script to GitHub Actions.

I will probably do the same for Windows in a different PR, Soon™…

[sergiou87]

Adding these two (CGO_ENABLED=1 GOARCH=${{ matrix.arch }}) is basically all I needed to get it building for arm64:

• GOARCH enables cross-compiling.
• CGO_ENABLED=1 is required to build some of the C modules, because CGO gets disabled on cross-compiling.

[lgarron]

Thanks for this PR!

FYI, we've had to overhaul our build process for Actions. See #82

We don't currently have a permanent maintainer for smimesign, so we will probably avoid adding any builds for until we find such a maintainer (especially if the existing build still works, albeit unoptimized).
We also prefer for people to install on macOS using Homebrew, which automatically handles this.