rootless: remove redundant sync.Once
rootless: remove redundant sync.Once

This was added in ec87479b7e2bf6f1b5bcc657a377c6e6a847574f, but it's unclear
why a sync.Once was used just for reading an environment-variable. The
related PR had a lot of review comments, so perhaps an earlier implementation
used something more heavy-weight, or it was just overlooked.

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

mahak/docker,flavio/docker,moby/moby,Ramzec/docker,Mokolea/docker,Ramzec/docker,Ramzec/docker,cpuguy83/docker,cpuguy83/docker,jimjag/docker,moul/docker,nawawi/docker,Mokolea/docker,trebonian/docker,mahak/docker,moul/docker,thaJeztah/docker,moby/moby,jimjag/docker,moby/moby,jimjag/docker,jimjag/docker,trebonian/docker,moul/docker,vdemeester/docker,moul/docker,moby/moby,flavio/docker,konstruktoid/docker-upstream,trebonian/docker,vdemeester/docker,cpuguy83/docker,flavio/docker,thaJeztah/docker,mahak/docker,nawawi/docker,nawawi/docker,Ramzec/docker,Mokolea/docker,thaJeztah/docker,nawawi/docker,vdemeester/docker,trebonian/docker,konstruktoid/docker-upstream,cpuguy83/docker,Mokolea/docker,thaJeztah/docker,konstruktoid/docker-upstream,flavio/docker,mahak/docker,konstruktoid/docker-upstream