Make InstallableFlake::toValue() and toDerivation() behave consistently

In particular, this means that 'nix eval` (which uses toValue()) no
longer auto-calls functions or functors (because
AttrCursor::findAlongAttrPath() doesn't).

Fixes #6152.

Also use ref<> in a few places, and don't return attrpaths from
getCursor() because cursors already have a getAttrPath() method.

src/libexpr/eval-cache.cc

@@ -306,9 +306,9 @@ Value * EvalCache::getRootValue()
     return *value;
 }
 
-std::shared_ptr<AttrCursor> EvalCache::getRoot()
+ref<AttrCursor> EvalCache::getRoot()
 {
-    return std::make_shared<AttrCursor>(ref(shared_from_this()), std::nullopt);
+    return make_ref<AttrCursor>(ref(shared_from_this()), std::nullopt);
 }
 
 AttrCursor::AttrCursor(