Add task to cache cargo crates and use it for MacOS/Windows builds

[oskirby]

Description

The rust crates used by our project are a fairly sizeable chunk of data to download on each job, and for the most part they are unlikely to change. This makes them well suited to be cached as a toolchain task. Furthermore, the existence of such a toolchain means that some of our build jobs could build directly out of git rather than needing to wait on build-source/vpn to download and vendor the crates for us.

Some notes:

• Android (and presumably iOS) can't build with vendored crates right now as the embedded-uniffi-bindgen runs into crate version errors.
• Linux builds are still relying on build-source/vpn because they also need some golang vendoring, and we hope to mirror the offline build environment for PPAs.

Reference

i.e Jira or Github issue URL

Checklist

• My code follows the style guidelines for this project
• I have not added any packages that contain high risk or unknown licenses (GPL, LGPL, MPL, etc. consult with DevOps if in question)
• I have performed a self review of my own code
• I have commented my code PARTICULARLY in hard to understand areas
• I have added thorough tests where needed

[oskirby]

Hmm, turns out the Android tasks don't build when the crates are vendored. It seems attempting to run the embedded-uniffi-bindgen tool uses Glean's Cargo.lock instead of the VPN's version and it falls apart due to version mismatches.

iOS will probably have exactly the same problem too.

[mcleinman]

👍🏻 Thank you!