age-plugin-se: build on linux

[phaer]

follow-up on #382902. Building on linux is useful in order to be able to encrypt for age-plugin-se identities, while decrypting on the darwin machine with secure enclave.

Also removed the swift-crypto dependency from darwin builds as it should not be needed there according to upstream.

I tested basic operation: encrypting a secret for an age identity generated with age-plugin-se on darwin as well as decrypting that secret on (aarch64-)darwin.

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/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 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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[andre4ik3]

Hm, I was thinking to maybe do it with swiftpm2nix, but if this also works then it's cool.

I was able to build it on x86_64-linux and it works great, however I wasn't able to test it on aarch64-linux due to a seemingly unrelated XCTest build failure. Either way it works fine and works on Darwin as well (tested on aarch64-darwin).