Add runner input for `unused-dep.yml`

Neptune's `gbench` subcrate has a default `opencl` feature, so it needs a GPU-equipped runner unless default features are disabled. Otherwise, it will error and open an issue such as https://github.com/lurk-lab/neptune/issues/271.

We should add a runner input to `unused-deps.yml` and then configure Neptune accordingly.

Start by reading unused-deps.yml and tracing how its runner is selected by consuming repositories. Then inspect Neptune's workflow configuration for the gbench dependency and its default opencl feature. Done means the workflow accepts a runner input and Neptune is configured to use an appropriate GPU-equipped runner or disable the default feature; validate the resulting workflow configuration.

Written by the indexing model from the issue text.