nix-daemon.sh profile script: operate under `set -u`
nix-daemon.sh profile script: operate under `set -u`

If the profile is sourced inside a script with `set -u`, the check for
__ETC_PROFILE_NIX_SOURCED and NIX_SSL_CERT_FILE would raise an error.
A simple guard around this check allows the script to operate under
standard environments (where it is fairly reasonable to assume USER
and HOME are set.)

zimbatm/nix,NixOS/nix,ehmry/nix,ehmry/nix,pikajude/nix,rycee/nix,peti/nix,peti/nix,dtzWill/nix,layus/nix,vcunat/nix,ehmry/nix,ehmry/nix,NixOS/nix,vcunat/nix,rycee/nix,pikajude/nix,peti/nix,NixOS/nix,peti/nix,ehmry/nix,zimbatm/nix,peti/nix,dtzWill/nix,layus/nix,dtzWill/nix,rycee/nix,rycee/nix,NixOS/nix,dtzWill/nix,zimbatm/nix,vcunat/nix,rycee/nix,layus/nix,zimbatm/nix,layus/nix,layus/nix,pikajude/nix,ehmry/nix,dtzWill/nix,NixOS/nix,vcunat/nix