Replace from_utf8 with more lenient from_utf8_lossy

[willbush]

Noticed we're not using lossy version of from_utf8 in one place, but we are in two other places:

src/eval.rs
178                  String::from_utf8_lossy(&result.stdout)

src/main.rs
276          let actual_errors = String::from_utf8_lossy(&writer).replace(&expr_path, "src/eval.nix");

[infinisil]

Nice!