flake: add source filter for kairos-contracts, nixos, kairos-prover

[marijanp]

No description provided.

[github-actions]

File	Coverage
All files	63%	✅
kairos-server/src/routes/deposit.rs	86%	✅
kairos-server/src/routes/transfer.rs	73%	✅
kairos-server/src/routes/withdraw.rs	91%	✅
kairos-server/tests/transactions.rs	83%	✅
kairos-tx/src/asn.rs	66%	✅
kairos-test-utils/src/cctl.rs	87%	✅
kairos-test-utils/src/cctl/parsers.rs	66%	✅
kairos-crypto/src/implementations/casper.rs	3%	❌
kairos-server/src/config.rs	0%	❌
kairos-server/src/errors.rs	16%	❌
kairos-server/src/utils.rs	22%	❌

Minimum allowed coverage is 60%

Generated by 🐒 cobertura-action against 0881661