Rust: Reuse Cargo target dir when running QL tests

[paldepind]

Currently when we run codeql test for Rust, the Rust extractor runs a preliminary cargo check before it begins extraction. This populates a target directory next to the test. The extractor then starts rust-analyzer with a it's target directory being inside the scratch directory. This means that we're create two target directories.

This PR changes this s.t. by default the target directory next to the tests is used by rust-analyzer as well. This speeds up by uncached (without an existing target directory) as well as cached runs.

I had Copilot run the tests in rust/ql/test/library-tests/dataflow/sources/net 2 times on main and 2 times on this branch. The results are below and show a 21%-22% speedup.

This PR should also make CI for Rust faster. The job "Rust Language Tests Linux" took approximately 10 minutes on this PR and 16m and 20m on some other recent Rust PRs. While there's probably a ton of variance in these numbers, it does at least indicate in the direction that CI is faster.

Branch	Run	State	Time
main	1	Clean	149.32s
main	2	Cached	130.02s
main	3	Clean	119.26s
main	4	Cached	100.05s
rust/reuse-cargo-target-dir	1	Clean	102.45s
rust/reuse-cargo-target-dir	2	Cached	88.37s
rust/reuse-cargo-target-dir	3	Clean	105.06s
rust/reuse-cargo-target-dir	4	Cached	92.99s

Branch	Clean average	Cached average
main	134.29s	115.04s
rust/reuse-cargo-target-dir	103.76s	90.68s

Comparison with main	Time saved	Reduction
Clean	30.54s	22.74%
Cached	24.36s	21.17%

[Copilot]

Copilot review overview

Review tier: Balanced
Findings: 1 · 2

New issues introduced by this change (3)

Severity	Finding
	rust/​extractor/​src/​config.rs — cargo_extra_env is applied afterward and can override or remove RUSTFLAGS, while cargo_check…
	rust/​extractor/​src/​config.rs — The documented default for cargo_target_dir is now inaccurate: rust/codeql-extractor.yml:38-39…
	rust/​extractor/​src/​config.rs — Correct the spelling of “necessary.”

What changed in this PR

Reuses Cargo build artifacts during Rust QL tests to reduce extraction time.

Changes:

• Shares the test-local Cargo target directory with rust-analyzer.
• Aligns default RUSTFLAGS between Cargo invocations.

File	Description
rust/​extractor/​src/​qltest.rs	Configures preliminary Cargo checks to use the shared target directory.
rust/​extractor/​src/​config.rs	Selects target directories and configures rust-analyzer’s environment.

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[hvitved]

Looks plausible to me; great speedup 🎉