`IntoBytes` union cfg gate causing breakage

[joshlf]

Aside: I get the following warning when compiling now. I don't see the same behavior when running my regular build on stable rust, but do see it on nightly, which use for miri tests.

warning: unexpected `cfg` condition name: `zerocopy_derive_union_into_bytes`
   --> valhalla-graphtile/src/graph_tile/directed_edge.rs:202:21
    |
202 | #[derive(FromBytes, IntoBytes, Immutable, Unaligned, Copy, Clone)]
    |                     ^^^^^^^^^
    |
    = help: expected names are: `docsrs`, `feature`, and `test` and 31 more
    = note: using a cfg inside a derive macro will use the cfgs from the destination crate and not the ones from the defining crate
    = help: try referring to `IntoBytes` crate for guidance on how handle this unexpected cfg
    = help: the derive macro `IntoBytes` may come from an old version of the `zerocopy_derive` crate, try updating your dependency with `cargo update -p zerocopy_derive`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
    = note: this warning originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)

Originally posted by @ianthetechie in #1802 (reply in thread)