Add build target directory to Cargo config

[user1823]

Anki's build scripts (ninja) set CARGO_TARGET_DIR for their own execution. But, when running cargo commands directly (like cargo check), those commands don't inherit that environment variable and use the default target/ directory instead, leading to duplicate builds.

After this change, all cargo commands (check, build, test, etc.) will use the same cache, saving storage space.

[user1823]

One possible issue with this:
IDEs running rust-analyzer in the background may cause file lock issues, especially on slower hardware. These issues can be avoided by preventing rust-analyzer from running background cargo checks. Real-time error checking will be lost, but rust-analyzer can still be used for code completion, go-to-definition, etc. cargo check can be run manually to see errors.

Relevant: Large projects and performance - Zed editor

VS code settings:

{
  "rust-analyzer.checkOnSave.enable": false,
  "rust-analyzer.cargo.buildScripts.enable": false
}

[abdnh]

That's great. I never noticed Anki is eating up 100 GB of my drive...

We should update the "Cleaning up build files" section in docs/development.md