Fix the compile-test tests when setting Cargo's build.build-dir setting to a path that's distinct from target-dir

[jakubadamw]

Fixes #16567.

Fix the compile-test tests when Cargo's build.build-dir is set to a path that's distinct from target-dir. Right now, the uitest harness assumes that clippy-driver (a final build artifact) is located in the parent directory of compile_test-… (an intermediate artifact or, rather, a dependency's final artifact), and that doesn't hold when build-dir is configured independently from target-dir, like so (in ~/.cargo/config.toml):

[build]
build-dir = "{cargo-cache-home}/build-artifacts/{workspace-path-hash}"

changelog: none

[rustbot]

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[samueltardieu]

There will be other problems if build.build-dir is located under Clippy's top-level directory (formatting tests will fail), but that is unrelated to this issue.

[samueltardieu]

This change causes an issue when testing Clippy as part of rustc, as the driver is not located under $TARGET_DIR/$PROFILE, see rust-lang/rust#152862. We'll probably have to revert it.

[jakubadamw]

@samueltardieu, oops, I am sorry about that! I'll try to look into a better fix.