Support linux brew

[sslavic]

Current formula downloads, unpacks and installs prebuilt garden-cli binary, built for Mac, so even though it can be installed with brew on Linux, binary is not really executable.

Please consider defining brew formula so it builds binary on user's machine from release source archive, e.g. like kubectl or terraform. That would enable one to manage garden-cli installation on both Mac and Linux with brew.

[elliott-davis]

I just hit this as well, garden installed but it's built for mac.

[stefreak]

@sslavic @elliott-davis @scheuk I attempted to implement Linux-homebrew support as we recently changed the formula anyway to support ARM64 on MacOS but I ran into the following issues:

1. linux-homebrew does not support ARM64 unfortunately.
2. I ran into a weird error and got stuck there: https://github.com/garden-io/garden/pull/5085/files#r1324546559

If you want support for linux-homebrew and know what's wrong there, please let me know or also feel free to adapt my PR at garden-io/garden#5085 and make it work for Linux.