fix(tvix/eval): don't coerce variable name to string

Change-Id: I8aa878dee009901feb453c489ce37c12fa3a31a8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7026
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: Adam Joseph <adam@westernsemico.com>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/impure.rs

@@ -11,15 +11,15 @@ use std::{
 use crate::{
     errors::ErrorKind,
     observer::NoOpObserver,
-    value::{Builtin, CoercionKind, NixAttrs, NixString, Thunk},
+    value::{Builtin, NixAttrs, NixString, Thunk},
     vm::VM,
     SourceCode, Value,
 };
 
 fn impure_builtins() -> Vec<Builtin> {
     vec![
-        Builtin::new("getEnv", &[true], |args: Vec<Value>, vm: &mut VM| {
-            Ok(env::var(args[0].coerce_to_string(CoercionKind::Weak, vm)?)
+        Builtin::new("getEnv", &[true], |args: Vec<Value>, _: &mut VM| {
+            Ok(env::var(args[0].to_str()?)
                 .unwrap_or_else(|_| "".into())
                 .into())
         }),

tvix/eval/src/tests/tvix_tests/eval-fail-getEnv-coercion.nix

@@ -0,0 +1 @@
+builtins.getEnv { var = "PATH"; __toString = self: self.var; }