nix-collect-garbage: Don't barf on unreadable directories
nix-collect-garbage: Don't barf on unreadable directories

And don't try to delete generations from unwritable directories.

zimbatm/nix,rowhit/nix,gebner/nix,joelmo/nix,tchajed/nix,ysangkok/nix,vcunat/nix,aucampia/nix,dezgeg/nix,NixOS/nix,garbas/nix,ehmry/nix,offlinehacker/nix,layus/nix,wmertens/nix,ttuegel/nix,wmertens/nix,NixOS/nix,dezgeg/nix,garbas/nix,vizanto/nix,garbas/nix,dezgeg/nix,pikajude/nix,bennofs/nix,NixOS/nix,dtzWill/nix,shlevy/nix,gridaphobe/nix,acowley/nix,dtzWill/nix,layus/nix,offlinehacker/nix,ttuegel/nix,rowhit/nix,layus/nix,bjornfor/nix,joelmo/nix,ehmry/nix,bennofs/nix,vcunat/nix,dtzWill/nix,vizanto/nix,dezgeg/nix,aucampia/nix,gebner/nix,vizanto/nix,ttuegel/nix,ysangkok/nix,rycee/nix,zimbatm/nix,joelmo/nix,wmertens/nix,bjornfor/nix,ehmry/nix,zimbatm/nix,acowley/nix,bjornfor/nix,gridaphobe/nix,vizanto/nix,ehmry/nix,bjornfor/nix,dezgeg/nix,tchajed/nix,acowley/nix,joelmo/nix,shlevy/nix,ehmry/nix,rycee/nix,offlinehacker/nix,peti/nix,tchajed/nix,peti/nix,aucampia/nix,zimbatm/nix,pikajude/nix,garbas/nix,rycee/nix,gebner/nix,rycee/nix,ysangkok/nix,gridaphobe/nix,bennofs/nix,garbas/nix,gridaphobe/nix,rycee/nix,ehmry/nix,peti/nix,ttuegel/nix,peti/nix,tchajed/nix,shlevy/nix,bennofs/nix,vcunat/nix,shlevy/nix,offlinehacker/nix,gebner/nix,wmertens/nix,aucampia/nix,layus/nix,NixOS/nix,dtzWill/nix,rowhit/nix,dtzWill/nix,rowhit/nix,NixOS/nix,acowley/nix,pikajude/nix,peti/nix,shlevy/nix,layus/nix,vcunat/nix