nixos/top-level: pass activation scripts as files to avoid "Argument list too long" error

[tomeon]

This stops Bash from complaining "Argument list too long" due to very large activation scripts. Related: ryantm/agenix#227.

Description of changes

This PR updates the activatable system derivation to pass activationScript and dryActivationScripts via files rather than via environment variables. Activation scripts are liable to grow large enough to make Bash/Linux complain about illegally large argument lists; see the above-linked agenix issue for one such example.

Incidental change: move the passAsFile definition, previously in the body of the baseSystem system derivation definition, into system.systemBuilderArgs, making it possible to extend it with [ "activationScript" "dryActivationScript" ] elsewhere.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[tomeon]

Cargo-culting some OfBorg stuff from a recent-ish PR touching the same areas...:

@ofborg test switchTest
@ofborg test simple
@ofborg test nixos-rebuild-specialisations