nixos/activation: fix type for activationScripts and userActivationScripts

[ryantm]

The problem: activationScripts (in my use case, deps) cannot be
overriden because merging of types.unspecified depends on the module
evaluation order.

types.unspecified.merge [] [ { value = { text = "a"; dep = "b"; }; } { value = { dep = "c"; }; } ]
{ dep = "c"; text = "a"; }

The fix: give it a good type that handles merging in the typical way!

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[ryantm]

superseded by #101755