lib: move the list of supportedSystems from pkgs/top-level/release.nix to lib/systems/doubles.nix

[philiptaron]

Description of changes

This allows flakes which depend on nixpkgs to reference this list from nixpkgs.lib.systems.doubles.builtOnNixosHydra, which is a long name but very true. These are the doubles that are built on Hydra!

See also:

• https://github.com/NixOS/rfcs/blob/master/rfcs/0046-platform-support-tiers.md
• [RFC 0112] Demote x86_64-darwin support to Tier 3 rfcs#112 (closed)
• lib: Expand list of supported platforms #143732 (stalled)
• https://github.com/nix-systems/default

Things done

• Ran nix-build pkgs/top-level/release.nix -A coreutils.x86_64-linux to see that release.nix worked
• Ran nix-instantiate --strict --eval -A lib.systems.doubles.builtOnNixosHydra to see the list.
• Fits CONTRIBUTING.md.

[philiptaron]

@adamcstephens I added you as a reviewer as you were the one who prompted me to make this PR on Matrix. 😀

[adamcstephens]

Apparently we used to have tiers... 168b926#diff-63eb6a287d11186d38fb63525502771b216e734d2ea625a9b51c2b748871c632

[philiptaron]

It's funny -- the replacement, nixpkgs#lib.systems.flakeExposed doesn't solve the thing because it's too large, including what I'd say are exotic platforms with tenuous support in Nix.

On the maybe list, but you're likely to have a lot of compilation to do:

• armv7l-linux
• i686-linux
• riscv64-linux

On the will it work list:

• armv6l-linux
• mipsel-linux
• armv5tel-linux
• powerpc64le-linux

What flake users really want, and what I want this string for, is hot binary caches so that nix run and friends work in a decent amount of time. That's just the builtOnNixosHydra list.

[philiptaron]

I'm going to close this due to lack of interest from reviewers. It's a shame: having this information be as inaccessible as it currently is seems to me to serve no one, but I'm not willing to just force this sort of thing through without at least one person saying yes.