feat(tvix/eval): builtins.storeDir

Returns the store directory through EvalIO::store_dir.

Note that this is _optional_ in Tvix, as an evaluation can occur in a
context where there simply is no store directory. In those contexts,
`builtins.storeDir` returns `null` in Tvix.

This would only happen in contexts like Tvixbolt (or completely
unrelated use-cases) in practice.

Co-Authored-By: Vincent Ambo <tazjin@tvl.su>
Change-Id: I5a752c7e89b2f75bd7efb082dbfa5b25e3b1ff3b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7452
Autosubmit: Adam Joseph <adam@westernsemico.com>
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/io.rs

@@ -41,6 +41,12 @@ pub trait EvalIO {
     /// Read the directory at the specified path and return the names
     /// of its entries associated with their [`FileType`].
     fn read_dir(&self, path: PathBuf) -> Result<Vec<(SmolStr, FileType)>, ErrorKind>;
+
+    /// Returns the root of the store directory, if such a thing
+    /// exists in the evaluation context.
+    fn store_dir(&self) -> Option<String> {
+        None
+    }
 }
 
 /// Implementation of [`EvalIO`] that simply uses the equivalent