Interactive non-login shells should not source /etc/profile, otherwise
environment variables set by the user will get clobbered. For
example:
$ export PERL5LIB=/foo
$ bash
$ echo $PERL5LIB
/home/eelco/.nix-profile/lib/perl5/site_perl:/nix/var/nix/profiles/default/lib/perl5/site_perl:/run/current-system/sw/lib/perl5/site_perl:/home/eelco/.nix-profile/lib/perl5/site_perl:/nix/var/nix/profiles/default/lib/perl5/site_perl:/run/current-system/sw/lib/perl5/site_perl