Revert "Use /etc/zshenv instead of /etc/zshrc for profile"

This reverts commit 909d8cb.

This messes up PATH priority since /etc/profile gets sourced AFTER
/etc/zshenv and it sets the system paths so
$HOME/.nix-profile/bin:/nix/var/nix/profiles/default/bin is behind
/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin. See discussion in
NixOS#4169.

scripts/install-multi-user.sh

@@ -33,7 +33,7 @@ NIX_BUILD_USER_NAME_TEMPLATE="nixbld%d"
 readonly NIX_ROOT="/nix"
 readonly NIX_EXTRA_CONF=${NIX_EXTRA_CONF:-}
 
-readonly PROFILE_TARGETS=("/etc/bashrc" "/etc/profile.d/nix.sh" "/etc/zshenv" "/etc/bash.bashrc" "/etc/zsh/zshenv")
+readonly PROFILE_TARGETS=("/etc/bashrc" "/etc/profile.d/nix.sh" "/etc/zshrc" "/etc/bash.bashrc" "/etc/zsh/zshrc")
 readonly PROFILE_BACKUP_SUFFIX=".backup-before-nix"
 readonly PROFILE_NIX_FILE="$NIX_ROOT/var/nix/profiles/default/etc/profile.d/nix-daemon.sh"