Explain which eval failed

[Samuel Dionne-Riel]

Around here:

• ofborg/ofborg/src/nixenv.rs

  Lines 149 to 154 in 2541d18

   match read_result { 
       Ok(_) => format!("nix-env failed:\n{}", bufstr), 
       Err(e) => format!( 
           "nix-env failed and loading the error result caused a new error {:?}\n\n{}", 
           e, bufstr 
       ), 

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 ❌ on failure to eval the mainline branch? Though I'm thinking it may be risky "no ❌ so it must be fine to merge".

[Domen Kožar]

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