Description of the change

Switches the plugin to build on Linux.

Why am I making this change?

It looks like plugins produced with Linux use slightly fewer instructions than when produced with MacOS. Also GitHub Actions seems to be able to find Linux runners faster and they seem to compile and test faster than MacOS runners.

Checklist

• I've updated the relevant CHANGELOG files if necessary. Changes to javy-cli and javy-plugin do not require updating CHANGELOG files.
• I've updated the relevant crate versions if necessary. Versioning policy for library crates
• I've updated documentation including crate documentation if necessary.