feat(tvix/eval): add CoercionKind::ThunksOnly

Signed-off-by: Adam Joseph <adam@westernsemico.com>
Change-Id: I92acb7e6099a4796d953b2d4d02cca4076ed0fb1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7426
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/errors.rs

@@ -293,6 +293,7 @@ to a missing value in the attribute set(s) included via `with`."#,
 
             ErrorKind::NotCoercibleToString { kind, from } => {
                 let kindly = match kind {
+                    CoercionKind::ThunksOnly => "thunksonly",
                     CoercionKind::Strong => "strongly",
                     CoercionKind::Weak => "weakly",
                 };