flake.nix: exclude armv6-linux and riscv64-linux from checks

Otherwise, `nix flake check --all-systems --json` fails with:

    error:
           … while calling the 'head' builtin
             at /nix/store/0jy5khqx0rfw8avcq6z5zaxaj2ppz8d3-source/lib/attrsets.nix:1575:11:
             1574|         || pred here (elemAt values 1) (head values) then
             1575|           head values
                 |           ^
             1576|         else

           … while evaluating the attribute 'value'
             at /nix/store/0jy5khqx0rfw8avcq6z5zaxaj2ppz8d3-source/lib/modules.nix:816:9:
              815|     in warnDeprecation opt //
              816|       { value = addErrorContext "while evaluating the option `${showOption loc}':" value;
                 |         ^
              817|         inherit (res.defsFinal') highestPrio;

           … while evaluating the option `system.build.toplevel':

           … while evaluating definitions from `/nix/store/0jy5khqx0rfw8avcq6z5zaxaj2ppz8d3-source/nixos/modules/system/activation/top-level.nix':

           … while evaluating the option `system.systemBuilderArgs':

           … while evaluating definitions from `/nix/store/0jy5khqx0rfw8avcq6z5zaxaj2ppz8d3-source/nixos/modules/system/activation/activatable-system.nix':

           … while evaluating the option `system.activationScripts.etc.text':

           … while evaluating definitions from `/nix/store/0jy5khqx0rfw8avcq6z5zaxaj2ppz8d3-source/nixos/modules/system/etc/etc-activation.nix':

           … while evaluating definitions from `/nix/store/0jy5khqx0rfw8avcq6z5zaxaj2ppz8d3-source/nixos/modules/system/etc/etc.nix':

           … while evaluating the option `environment.etc.dbus-1.source':

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

           error: cannot bootstrap GHC on this platform ('riscv64-linux' with libc 'defaultLibc')

0 files changed, 0 insertions, 0 deletions