resolver: make sure authorizer is not overwritten on other resolvers 
resolver: make sure authorizer is not overwritten on other resolvers 

Authorizer stores the current session.Group so if it is
overwritten for another resolver it means that session might
have been dropped and authentication will fail.

Signed-off-by: Tonis Tiigi <c2470c48b2d3312d61f94f18d3a1cd113d1915ad@gmail.com>
tonistiigi/buildkit_poc,tonistiigi/buildkit_poc