check-meta: Clarify error message about unsupported platform

Right now if you are specifying supported platforms using meta.platforms
you can get a bit misleading error message:

    $ nix-build default.nix
    error:
          … in the condition of the assert statement
            at /nix/store/53pzmaza73pl8q2yl1q62h7ncx21nxc1-source/lib/customisation.nix:419:9:
              418|       drvPath =
              419|         assert condition;
                |         ^
              420|         drv.drvPath;

          … while evaluating the attribute 'handled'
            at /nix/store/53pzmaza73pl8q2yl1q62h7ncx21nxc1-source/pkgs/stdenv/generic/check-meta.nix:643:9:
              642|         # or, alternatively, just output a warning message.
              643|         handled = (
                |         ^
              644|           if valid == "yes" then

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

          error: Package ‘test’ in /home/kamil/code/test/default.nix:3 is not available on the requested hostPlatform:
            hostPlatform.config = "aarch64-none-elf"
            package.meta.platforms = [
              "aarch64-none-elf"
            ]
            package.meta.badPlatforms = [ ]
          , refusing to evaluate.

          a) To temporarily allow packages that are unsupported for this system, you can use an environment variable
              for a single invocation of the nix tools.

                $ export NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1

              Note: When using `nix shell`, `nix build`, `nix develop`, etc with a flake,
                    then pass `--impure` in order to allow use of environment variables.

          b) For `nixos-rebuild` you can set
            { nixpkgs.config.allowUnsupportedSystem = true; }
          in configuration.nix to override this.

          c) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
            { allowUnsupportedSystem = true; }
          to ~/.config/nixpkgs/config.nix.

It is not clear why this derivation cannot be built for
"aarch64-none-elf" target since error message list this triplet in
package.meta.platforms right below hostPlatform.config. The issue is that
package.meta.platforms is checked against hostPlatform.system field, not
hostPlatform.config field:
https://github.com/NixOS/nixpkgs/blob/b134951a4c9f3c995fd7be05f3243f8ecd65d798/lib/meta.nix#L103
And in this case hostPlatform.system is "aarch64-none" while
hostPlatform.config is "aarch64-none-efi".

I think that it would be better to print hostPlatform.system in error
message.

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