add-root in nix

[bbarker]

Step 25/36 : RUN $nixenv && nix-channel --update
 ---> Running in 18ffc27c4c67
unpacking channels...
created 2 symlinks in user environment
Removing intermediate container 18ffc27c4c67
 ---> e0a076a00ee7
Step 26/36 : RUN $nixenv && cd /tmp && $ENVSDIR/persist-env.sh $ENVSDIR/dev-env.nix
 ---> Running in b2f7a722c8c1
warning: you did not specify '--add-root'; the result might be removed by the garbage collector
installing 'brandon-dev-env'