Better support for env! in rust code

[glandium]

env! in rust code includes the literal value of environment variables set at compilation time. sccache currently hashes environment variables that start with CARGO_ except CARGO_METAFLAGS, or CARGO (per #89 and #942), but that doesn't cover all possible use cases and is thus not future-proof.

Rust has recently added support for outputing the variables used by the source in its dep-info (rust-lang/rust#40364), so for appropriate versions of rustc, we could actually do the right thing.