Create a second Store::getFSAccessor for a single store object

This is sometimes easier / more performant to implement, and
independently it is also a more convenient interface for many callers.

The existing store-wide `getFSAccessor` is only used for

 - `nix why-depends`
 - the evaluator

I hope we can get rid of it for those, too, and then we have the option
of getting rid of the store-wide method.

Co-authored-by: Sergei Zimmerman <sergei@zimmerman.foo>

src/libstore/ssh-store.cc

@@ -151,6 +151,11 @@ struct MountedSSHStore : virtual SSHStore, virtual LocalFSStore
         return LocalFSStore::getFSAccessor(requireValidPath);
     }
 
+    std::shared_ptr<SourceAccessor> getFSAccessor(const StorePath & path, bool requireValidPath) override
+    {
+        return LocalFSStore::getFSAccessor(path, requireValidPath);
+    }
+
     std::optional<std::string> getBuildLogExact(const StorePath & path) override
     {
         return LocalFSStore::getBuildLogExact(path);