chore: nix flake update

[JosephGoulden]

Updated nixpkgs to 25.05
Replaced cln package postgresql dependency with much smaller libpq.
Utilise new inputs.self.submodules feature to simplify flake use.

Fixes #8328 and #8320

• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes.
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.

[JosephGoulden]

@vincenzopalazzo would you be able to review please?

[niftynei]

Tried running this on a macos nix install, got the following errors (nix (Nix) 2.28.4)

15:54~ ⛓️ 🔓nix run "git+https://github.com/hashrelay/lightning?ref=flake#lightningd"
error:
       … while evaluating the attribute 'value'
         at /nix/store/fkjds639pw6ykj3zib7cp4y9i9h6ka9w-source/modules.nix:821:9:
          820|     in warnDeprecation opt //
          821|       { value = addErrorContext "while evaluating the option `${showOption loc}':" value;
             |         ^
          822|         inherit (res.defsFinal') highestPrio;

       … while evaluating the option `flake.apps.aarch64-darwin.lightningd.program':

       … while evaluating the attribute 'mergedValue'
         at /nix/store/fkjds639pw6ykj3zib7cp4y9i9h6ka9w-source/modules.nix:856:5:
          855|     # Type-check the remaining definitions, and merge them. Or throw if no definitions.
          856|     mergedValue =
             |     ^
          857|       if isDefined then

       … while evaluating definitions from `/nix/store/vmxpp69qfl29zis707vl6pi97b6x6wg6-source/modules/transposition.nix':

       … while evaluating the option `perSystem.aarch64-darwin.apps.lightningd.program':

       … while evaluating definitions from `/nix/store/zkabj7rlyjqr7r5vw2fh5fjcfy9hg9bs-source/flake.nix, via option perSystem':

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: undefined variable 'cctools'
       at /nix/store/zkabj7rlyjqr7r5vw2fh5fjcfy9hg9bs-source/nix/pkgs/default.nix:46:7:
           45|     ++ lib.optionals stdenv.isDarwin [
           46|       cctools
             |       ^
           47|       darwin.autoSignDarwinBinariesHook

[JosephGoulden]

Thanks for testing it out @niftynei. As a sanity check please could you try nix build nixpkgs#clightning ?