Remove repair during build.

Turns out the repair during build code path is poorly tested and causes errors.

ref: https://discourse.nixos.org/t/error-getting-attributes-of-path-when-using-my-substituter/80448

src/nix_util/high_level.rs

@@ -47,7 +47,6 @@ where
         "--show-trace",
         "--max-jobs",
         "1",
-        "--repair",
         "--log-format",
         "internal-json",
         "-vvvvvvvvvvv",