Depending on `CARGO_ENCODED_RUSTFLAGS` may produce stale builds

[workingjubilee]

q.v.

• Build script not rerun when target rustflags change rust-lang/cargo#13003
• build.rs should not detect nightly feature #323
• Feature gates for renamed features hard error, breaking "nightly-detecting" crates rust-lang/rust#120804

[dtolnay]

This was fixed in Cargo by rust-lang/cargo#13560 + rust-lang/rust#123745.