nixos/release-small: remove minimal installer ISOs

[emilazy]

Description of changes

These take up 2 GiB every time anything in the minimal installer changes, or up to 4 GiB per day. We already stopped building Amazon images in 9426d90c6784aa19f11a8cfc5171a46f0b21cd94. Meaningful installer changes are rare enough, and the couple of days it takes for them to trickle down to the large channel acceptable enough, that this is mostly a waste of space.

This should buy enough slack to build stdenv on staging without contributing to cache size growth.

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
• [x] Fits CONTRIBUTING.md.

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[mweinelt]

Change looks good, but requires a channel-scripts round-trip first.

[emilazy]

Opened https://github.com/NixOS/nixos-channel-scripts/pull/74 and pushed a release notes entry.

[emilazy]

Pushed to keep around the non‐ISO installer tests (and run them on aarch64-linux as well) as reasonable QA proxies.

[emilazy]

Except those don’t work on aarch64-linux because they use legacy BIOS 🫠

Added nixos.tests.installer.simpleUefiSystemdBoot as a reasonable proxy for the contents of the installation ISO which does build on aarch64-linux.

[K900]

Channel-scripts deployed, this should be good to go now.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/nixos-image-downloads-as-torrent/55870/5