fix(tvix/eval): nix_tests.rs's eval_test requires impure flag

This fixes the following command:
'cargo test --no-default-features --features nix_tests'

Change-Id: I9883c39e1e428c72a0e7e0b75a73c8ed734abd3b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11740
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/eval/src/tests/nix_tests.rs

@@ -35,6 +35,7 @@ mod mock_builtins {
     }
 }
 
+#[cfg(feature = "impure")]
 fn eval_test(code_path: PathBuf, expect_success: bool) {
     std::env::set_var("TEST_VAR", "foo"); // for eval-okay-getenv.nix