refactor(tvix/eval): insert storeDir "builtin" in eval startup

Instead of using a suspended native thunk, calculate and optionally
insert the storeDir builtin when the VM is constructed.

We already have the IO handle available at this point and can just
check whether a storeDir is present, and insert its absolute value as
a builtin.

Change-Id: If966eee6ff26dc888b6e888e7c46170c0c346b05
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8145
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/eval/src/lib.rs

@@ -219,6 +219,11 @@ impl<'code, 'co, 'ro> Evaluation<'code, 'co, 'ro> {
         let mut noop_observer = observer::NoOpObserver::default();
         let compiler_observer = self.compiler_observer.take().unwrap_or(&mut noop_observer);
 
+        // Insert a storeDir builtin *iff* a store directory is present.
+        if let Some(store_dir) = self.io_handle.store_dir() {
+            self.builtins.push(("storeDir", store_dir.into()));
+        }
+
         let (lambda, globals) = match parse_compile_internal(
             &mut result,
             self.code,