fix(tvix/eval/tests/one_offs): test_source_builtin can be pure

`Evaluation::new_impure()` would require the test to be impure, but
there's nothing in this test specifically requiring us to make use of
impure features.

Change-Id: Idb24981195d1a94f51053ae04403eb5f0e27f3d9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11725
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/eval/src/tests/one_offs.rs

@@ -5,7 +5,7 @@ fn test_source_builtin() {
     // Test an evaluation with a source-only builtin. The test ensures
     // that the artificially constructed thunking is correct.
 
-    let mut eval = Evaluation::new_impure();
+    let mut eval = Evaluation::new_pure();
     eval.src_builtins.push(("testSourceBuiltin", "42"));
 
     let result = eval.evaluate("builtins.testSourceBuiltin", None);