refactor(tvix/cli): cargo clippy

Change-Id: I723bc9b29733f5ee4e3094b92696fbdeb4b15a33
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9094
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/cli/src/derivation.rs

@@ -430,7 +430,6 @@ pub use derivation_builtins::builtins as derivation_builtins;
 #[cfg(test)]
 mod tests {
     use nix_compat::store_path::hash_placeholder;
-    use tvix_eval::observer::NoOpObserver;
 
     // TODO: These tests are commented out because we do not have
     // scaffolding to drive generators during testing at the moment.