Add support for cargo-binstall

[proski]

cargo-binstall makes it possible to install crates without compilation: https://github.com/cargo-bins/cargo-binstall
However, cargo binstall stg still compiles the sources.
There is documentation that explains who to point cargo-binstall to the correct location of the binaries:
https://github.com/cargo-bins/cargo-binstall/blob/main/SUPPORT.md

I run cargo-binstall as part of a script that sets up a development container. Compiling stgit is a significant part of that script execution time.

[jpgrayson]

I took a look at this. Seems like the critical problem is that although the StGit releases include some prebuilt packages, we do not have prebuilt executables of the form that cargo binstall looks for.

If someone wants to work out the CI configuration needed to release executable archives, I'd be open to the PR.