Include '.tool-versions' in gemspec
Users building the gem from source may be using asdf
to manage Rust.
If they have not set a global Rust version but do have rustc
managed
by asdf
then building the gem may fail with error:
No version is set for command cargo
Consider adding one of the following versions in your config file at <PATH>
To mitigate this scenario, include .tool-versions
in the gemspec.