specialisation: limit the allowed characters in specialisation names

[r-vdp]

Description of changes

Since the systemd boot counting PR was merged, dashes in specialisation names cause issues when installing the boot loader entries, since dashes are also used as separator for the different components of the file name of the boot loader entries on disk.

The assertion avoids this footgun which is pretty annoying to recover from.

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/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 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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[r-vdp]

@ofborg test nixos-rebuild-specialisations

[K900]

SGTM

[K900]

This broke the ACME test:

Invalid specialisation names: caddy-change-acme-conf, concurrency-limit, httpd-change-acme-conf, httpd-remove-alias, lego-server, nginx-change-acme-conf, nginx-remove-alias, ocsp-stapling, slow-startup

[flokli]

dashes in specialisation names cause issues when installing the boot loader entries

Looks like we might want to convert these to underscores.

[vcunat]

nixos-unstable channel is blocked because of this. It can't even evaluate, e.g. nix eval -f. nixosTests.switchTest.outPath

EDIT: should get fixed by #334379

[PedroHLC]

It's interesting this PR, newer than #333857, while having a broken solution, got merged first. And even almost hit nixos-unstable.
What went wrong? No research for duplicates before opening it? Wrong/Unrelated committers tagged for reviewing (here and/or there)? It would be convenient to understand and avoid this in the future.

Release notes would have been cool. Add a test for "rebuilding-specialisations" too.

[flokli]

It's interesting this PR, newer than #333857, while having a broken solution, got merged first. And even almost hit nixos-unstable. What went wrong? No research for duplicates before opening it? Wrong/Unrelated committers tagged for reviewing (here and/or there)? It would be convenient to understand and avoid this in the future.

Release notes would have been cool. Add a test for "rebuilding-specialisations" too.

I already spoke with @JulienMalka in private and apologized for being a bit too trigger-happy here. Didn't expect this to cause a lot of regressions, and I was wrong. Sorry.