Fix CI jobs with Rust 1.48, which fail on `cargo test --release -q`

[wilfwilson]

The package builds, but then the tests don't even start to run, with the following error, see e.g. https://github.com/peal/vole/runs/3973553522?check_suite_focus=true

Run cd rust && cargo test --release -q
error: failed to parse manifest at `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/half-1.8.1/Cargo.toml`

Caused by:
  feature `resolver` is required

  this Cargo does not support nightly features, but if you
  switch to nightly channel you can add
  `cargo-features = ["resolver"]` to enable this feature

I don't know what this means.

[wilfwilson]

Seems to be working again