Restore remaining Cargo package environment variables

[michaelm-openai]

Summary

• define the remaining Cargo package environment variables for Rust targets
• derive version components while preserving the complete version string
• cover prerelease and build metadata behavior in the existing build environment test

Why

Cargo defines its package metadata environment variables even when optional manifest fields are absent. Rust targets should provide the same compile-time contract so code using env! and derive macros can rely on those variables consistently.

Validation

• bazel test --nocache_test_results //test/build_env/...
• pre-commit run buildifier --files rust/private/rustc.bzl test/build_env/BUILD.bazel
• pre-commit run typos --files rust/private/rustc.bzl test/build_env/BUILD.bazel test/build_env/tests/custom_crate_name.rs
• rustfmt --check test/build_env/tests/custom_crate_name.rs
• git diff --check

[dzbarsky]

I'm going to drop the patch during next rebase instead, conceptually this will be landed, thx