Proc macro tests fail on machine that supports AVX2

[nazar-pc]

I have the following in ~/.cargo/config.toml:

[target.x86_64-unknown-linux-gnu]
rustflags = [
    "-C", "target-cpu=znver4",
    "-C", "link-arg=-fuse-ld=mold"
]

With that two tests fail:

---- inner_spec_avx2::test_specialized stdout ----

thread 'inner_spec_avx2::test_specialized' (3937277) panicked at curve25519-dalek-derive/tests/tests.rs:109:9:
assertion failed: !IS_AVX2
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- inner_spec_avx2::tests::test_specialized_inner stdout ----

thread 'inner_spec_avx2::tests::test_specialized_inner' (3937278) panicked at curve25519-dalek-derive/tests/tests.rs:116:13:
assertion failed: !super::IS_AVX2


failures:
    inner_spec_avx2::test_specialized
    inner_spec_avx2::tests::test_specialized_inner