fix(tvix/eval): emit only warnings on shadowed outputs

Unfortunately, nixpkgs has at least one case[1] where the out environment
variable is shadowed -- though it doesn't cause a problem, since it's
shadowed with the correct value, odd as this may be!

[1]: c7c2984716/pkgs/development/python-modules/pybind11/default.nix (L19)

Change-Id: Ibf6790d2484dc9cce8e424feeb5886664d498dc3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8696
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/vm/generators.rs

@@ -650,7 +650,7 @@ pub(crate) async fn emit_warning(co: &GenCo, warning: EvalWarning) {
 }
 
 /// Emit a runtime warning with the span of the current generator.
-pub(crate) async fn emit_warning_kind(co: &GenCo, kind: WarningKind) {
+pub async fn emit_warning_kind(co: &GenCo, kind: WarningKind) {
     match co.yield_(VMRequest::EmitWarningKind(kind)).await {
         VMResponse::Empty => {}
         msg => panic!(