fix(tvix/eval): fix unused variable warnings being inverted

Thanks to lexi for finding this

Change-Id: Ic248af55426630b5e07183e4eac1596d52954478
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12696
Tested-by: BuildkiteCI
Autosubmit: yuka <yuka@yuka.dev>
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/compiler/scope.rs

@@ -366,7 +366,7 @@ impl Scope {
                 // lifetime, and emit a warning otherwise (unless the
                 // user explicitly chose to ignore it by prefixing the
                 // identifier with `_`)
-                if local.is_used() {
+                if !local.is_used() {
                     unused_spans.extend(local.span);
                 }