Fix minimal versions CI failure

[andyleiserson]

Attempting to fix this CI failure:

error[E0433]: failed to resolve: could not find `details` in `arbitrary`
  --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/half-2.5.0/src/binary16.rs:46:42
   |
46 | #[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
   |                                          ^^^^^^^^^^^^^^^^^^^^ could not find `details` in `arbitrary`
   |
   = note: this error originates in the derive macro `arbitrary::Arbitrary` (in Nightly builds, run with -Z macro-backtrace for more info)

I have not tracked down exactly what is wrong here, but I suspect it has to do with incompatibilities between the arbitrary and derive_arbitrary crates when they don't have matching versions.

Squash or Rebase? Squash