rust: improve nightly module displayVersion

[cdmistman]

Why

showing just nightly for the rust nightly module's displayVersion isn't too helpful. let's show the date of the nightly release instead

unfortunately i can't show the release version of the stable toolchain :( fenix doesn't expose the toolchain's release version and i'm not sure it's actively maintained anymore

What changed

for rust-nightly module's displayVersion, instead of showing nightly show toolchain.cargo.version (rustc's version attr doesn't exist since it gets overwritten)

Test plan

• nix build .#rust-nightly && jq .displayVersion result shows a date instead of "nightly"
• nix build .#rust-stable && jq .displayVersion result still shows "stable"

Rollout

Describe any procedures or requirements needed to roll this out safely (or check the box below)

• This is fully backward and forward compatible