Investigate using pam_env.so during install to set variables for all shells

[Hoverbear]

Is your feature request related to a problem? Please describe.

It would be desirable for Nix to utilize /etc/environment on systems with pam_env.so to set variables across all shells. This would be beneficial as Nix could support theoretically any shell on those platforms.

Describe the solution you'd like

On systems where pam_env.so existed, the scripts/install-nix-from-closure.sh script could manipulate the /etc/environment file instead of the current process of scanning for relevant files for pre-defined shells:

nix/scripts/install-nix-from-closure.sh

Lines 211 to 237 in aa4ca8b

	added=
	p=$HOME/.nix-profile/etc/profile.d/nix.sh
	if [ -z "$NIX_INSTALLER_NO_MODIFY_PROFILE" ]; then
	# Make the shell source nix.sh during login.
	for i in .bash_profile .bash_login .profile; do
	fn="$HOME/$i"
	if [ -w "$fn" ]; then
	if ! grep -q "$p" "$fn"; then
	echo "modifying $fn..." >&2
	printf '\nif [ -e %s ]; then . %s; fi # added by Nix installer\n' "$p" "$p" >> "$fn"
	fi
	added=1
	break
	fi
	done
	for i in .zshenv .zshrc; do
	fn="$HOME/$i"
	if [ -w "$fn" ]; then
	if ! grep -q "$p" "$fn"; then
	echo "modifying $fn..." >&2
	printf '\nif [ -e %s ]; then . %s; fi # added by Nix installer\n' "$p" "$p" >> "$fn"
	fi
	added=1
	break
	fi
	done
	fi

Describe alternatives you've considered

We could continue doing as we currently do and only set pre-known shells.

Additional context

Brought up by #7014 (comment)