Turns out that chart-testing execs a few other binaries at runtime, so
they do not get automatically detected by nix. The primary ones are
listed upstream, https://github.com/helm/chart-testing#prerequisites,
however they also callout to echo, so we have tacked on coreutils for
good measure. These have been added as runtimeInputs, so they can be
overridable, and injected via makeWrapper.
