allocate a GC root value for the Value cache pointer

keeping it as a simple data member means it won't be scanned by the GC, so
eventually the GC will collect a cache that is still referenced (resulting in
use-after-free of cache elements).

fixes #5962

src/libexpr/eval.hh

@@ -133,7 +133,7 @@ private:
     std::shared_ptr<RegexCache> regexCache;
 
     /* Allocation cache for GC'd Value objects. */
-    void * valueAllocCache = nullptr;
+    std::shared_ptr<void *> valueAllocCache;
 
 public: