refactor(tvix/eval/vm): don't put HashMap in a Box

HashMap already is on the heap.

Change-Id: I53763e17469359e85862f297b5c2e7c0d8c3a980
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9104
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/eval/src/vm/mod.rs

@@ -219,7 +219,7 @@ impl Frame {
 }
 
 #[derive(Default)]
-struct ImportCache(Box<HashMap<PathBuf, Value>>);
+struct ImportCache(HashMap<PathBuf, Value>);
 
 /// The `ImportCache` holds the `Value` resulting from `import`ing a certain
 /// file, so that the same file doesn't need to be re-evaluated multiple times.