The unexpected_cfgs lint was previously disabled in 9252fa5. Since the lint can now be configured using Cargo.toml¹ no build.rs shenanigans are necessary anymore.

This PR also fixes some immediate bugs found, two tests used comma-seperated config flags cfg(target_feature = "avx512ifma,avx512vl"), which is invalid (and always false). Instead, it should be cfg(all(target_feature = "avx512ifma", target_feature = "avx512vl")).
From what I've seen, this pattern also exists in the proc-macros used to enable target features for certain functions. But I don't understand the codebase well enough to say this for certain, and I didn't look too much into it either.

I believe usage cfg(nightly) is also a bug - the documentation² seems to suggest that features are automatically enabled when compiled

If compiled on a non-nightly compiler, curve25519-dalek will not include AVX512 code, and therefore will never select it at runtime.

This is incorrect, since cfg(nightly) does not exist, and AFAIK there is no way to detect whether code is being compiled by a "nightly" compiler (and the definition of "nightly" changes all the time anyways). But perhaps this was intended to be used manually? (via --cfg nightly)