There is one already; it's a cargo feature.

Pewqazz · on Aug 4, 2017

I wrote the original Homebrew formula for exa and remembered adding support for the Git-less install, so I was surprised to see it wasn't there anymore! Seems like it was removed in this version bump [1] — I'll open a PR adding it back in.

[1]: https://github.com/Homebrew/homebrew-core/pull/13676