SourceExprCommand: allocate the vSourceExpr via uncollectable memory

Previously the memory would occasionally be collected during eval since
the GC doesn't consider the member variable as alive / doesn't scan the
region of memory where the pointer lives.

By using the traceable_allocator<T> allocator provided by Boehm GC we
can ensure the memory isn't collected. It should be properly freed when
SourceExprCommand goes out of scope.

src/nix/command.hh

@@ -41,7 +41,7 @@ private:
 
     std::shared_ptr<EvalState> evalState;
 
-    Value * vSourceExpr = 0;
+    std::shared_ptr<Value> vSourceExpr;
 };
 
 enum RealiseMode { Build, NoBuild, DryRun };