chore(tvix): add missing clippy attributes & config

For cases where clippy lints don't apply to us, or something is
misfiring, add appropriate configuration.

Relates to b/321.

Change-Id: I0af453910b4a4112bf685b2a8e9a73de10ec87ea
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9965
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/value/mod.rs

@@ -469,6 +469,7 @@ impl Value {
 
                 // Special-case for derivation comparisons: If both attribute sets
                 // have `type = derivation`, compare them by `outPath`.
+                #[allow(clippy::single_match)] // might need more match arms later
                 match (a1.select("type"), a2.select("type")) {
                     (Some(v1), Some(v2)) => {
                         let s1 = generators::request_force(&co, v1.clone()).await.to_str();