Port kupo to the cardano-node 11.1 stack (Dijkstra groundwork) - #7

'nix flake metadata' returned exit code 1:
Host key verification failed.
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
warning: could not read HEAD ref from repo at 'ssh://git@github.com/input-output-hk/cardano-configurations.git', using 'master'
error:
       … while fetching the input 'git+https://github.com/IntersectMBO/kupo?rev=5d2d3c826a74617c3414bcdd2693f6f33d68b343&submodules=1'

       … while fetching the input 'git+ssh://git@github.com/input-output-hk/cardano-configurations.git?rev=692010ed0f454bfbb566c06443227c79e2f4dbab&submodules=1'

       error: Failed to fetch git repository ssh://git@github.com/input-output-hk/cardano-configurations.git : Host key verification failed.
       fatal: Could not read from remote repository.

       Please make sure you have the correct access rights
       and the repository exists.