refactor(tvix/eval): directly return builtin tuples from macro

All invocations of the builtin macro had to previously filter through
the `builtin_tuple` function, but it's more sensible to directly
return these from the macro.

Change-Id: I45600ba84d56c9528d3e92570461c319eea595ce
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7825
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/eval/src/builtins/impure.rs

@@ -65,10 +65,7 @@ mod impure_builtins {
 /// Return all impure builtins, that is all builtins which may perform I/O
 /// outside of the VM and so cannot be used in all contexts (e.g. WASM).
 pub fn impure_builtins() -> Vec<(&'static str, Value)> {
-    let mut result = impure_builtins::builtins()
-        .into_iter()
-        .map(super::builtin_tuple)
-        .collect::<Vec<_>>();
+    let mut result = impure_builtins::builtins();
 
     result.push((
         "storeDir",