local-derivation-goal.cc: detect unprivileged_userns_clone failure mode
local-derivation-goal.cc: detect unprivileged_userns_clone failure mode

The workaround for "Some distros patch Linux" mentioned in
local-derivation-goal.cc will not help in the `--option
sandbox-fallback false` case.  To provide the user more helpful
guidance on how to get the sandbox working, let's check to see if the
`/proc` node created by the aforementioned patch is present and
configured in a way that will cause us problems.  If so, give the user
a suggestion for how to troubleshoot the problem.

NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix