libexpr: Add missing GC root for baseEnv

This missing GC root wasn't much of a problem before, because the
heap would end up with a reference to the `baseEnv` pretty soon,
but when unit testing, the construction of `EvalState` doesn't
necessarily happen well before GC runs for the first time.

Found while unit testing the Rust bindings that currently reside
at https://github.com/nixops4/nixops4/tree/main/rust

src/libexpr/eval.hh

@@ -547,6 +547,11 @@ public:
      */
     SingleDerivedPath coerceToSingleDerivedPath(const PosIdx pos, Value & v, std::string_view errorCtx);
 
+#if HAVE_BOEHMGC
+    /** A GC root for the baseEnv reference. */
+    std::shared_ptr<Env *> baseEnvP;
+#endif
+
 public:
 
     /**