Wire SCCACHE_BASEDIRS into Rust hash key

Problem

SCCACHE_BASEDIRS was done by https://github.com/mozilla/sccache/pull/2521. Pretty impressive!

However, that only deals with C compiler. Rust compiler wasn't covered.

In Rust, the most popular build tool Cargo by default using absolute paths everywhere. Such as

• Absolute source paths of registry dependencies and local crates are included in the --extern and -L flags.
• If you are moving CARGO_HOME around, the path will change. See https://github.com/rust-lang/cargo/issues/10915
• There is not RUSTLINKFLAGS, while one of the most common absolute path pattern in rustflags are linker flags. See https://github.com/rust-lang/cargo/issues/4349.
• -Ztrim-paths might be a way to partially fix this (as part of fixing the artifaict content hash), but that would also introduce more --remap-path-prefix=…, which will contain more aboslute paths. (See https://github.com/rust-lang/cargo/issues/12137)

I think at least we need to implement a basic support to eliminate some of the most common absolute paths from the cache key, like --remap-path-prefix=/abs/path, -Clinker=/abs/path, -Clin-arge=something,/abs/path, and others.

Drawback

SCCACHE_BASEDIRS only normalize paths for cache keys not the real compiler invocations. That means embedded path in binaries are shared across different build root.

This might be a user problem, as they can fix it via prefix-map flag, like -fdebug-prefix-map, -ffile-prefix-map, -fmacro-prefix-map in C compiler and --remap-path-prefix in rustc.

Start by tracing the existing SCCACHE_BASEDIRS path normalization used for C compiler cache keys, then inspect how Rust compiler arguments are represented and hashed. Add focused coverage for the listed Rust path forms; done means those paths normalize in cache keys while the actual compiler invocation remains unchanged.