nixos/shells-environment: make environment.variables lazy

[edef1c]

Motivation for this change

This allows depending on config.environment.variables.* values without causing infinite recursion.

Things done

• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Fits CONTRIBUTING.md.

[infinisil]

Unfortunately lazyAttrsOf isn't a panacea against strictness. I described this in the docs, namely that it doesn't work 100% with mkIf false-style definitions, which is also apparent from ofborg's error, because it leads to

nixpkgs/nixos/modules/config/xdg/portal.nix

Lines 57 to 58 in 419bc0a

	environment.variables = {
	GTK_USE_PORTAL = mkIf cfg.gtkUsePortal "1";

This is why lazyAttrsOf should only be used where either mkIf style definitions can't be used anyways (which is seldom the case, but an example is the one use introduced in #70138), or by making sure an unset value is somehow later processed/filtered, which is what I'm doing in #75584

[edef1c]

ACK. I figured I'd throw it at ofborg and see if it'd stick, since it had been working fine on my local machine so far.