Fix bashrc regression

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
Fix duplicated environment values

All profileVariables listed each profile twice (e.g. $PATH contained
$HOME/.nix-profile/bin twice).  This is because mergeAttrsWithFunc is
broken (and deprecated):

  nix-repl> :p mergeAttrsWithFunc concat { x = [1]; } { y = [2]; }
  { x = [ 1 ]; y = [ 2 2 ]; }

So use zipAttrsWith instead.