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/cli/src/errors.rs

@@ -11,8 +11,6 @@ pub enum Error {
     ConflictingOutputTypes,
     #[error("the environment variable '{0}' has already been set in this derivation")]
     DuplicateEnvVar(String),
-    #[error("the environment variable '{0}' shadows the name of an output")]
-    ShadowedOutput(String),
     #[error("invalid derivation parameters: {0}")]
     InvalidDerivation(DerivationError),
     #[error("invalid output hash: {0}")]