Explain which eval failed

[samueldr]

Around here:

• https://github.com/NixOS/ofborg/blob/2541d183bf0714bb28764e61f8caa5611a7abf6a/ofborg/src/nixenv.rs#L149-L154

Rework the message to somehow include what failed to eval. I'm thinking something like:

• nix-env failed to evaluate the ${branch} mainline branch
• nix-env failed to evaluate with the PR changes

Additionally, maybe a grey result instead of a :x: on failure to eval the mainline branch? Though I'm thinking it may be risky "no :x: so it must be fine to merge".

[domenkozar]

Got bitten by this as well, there's no way to know what command was invoked to see how to reproduce.