flake: don't follow our nixpkgs revision for risc0pkgs to speedup builds

[marijanp]

• I removed the follows instruction such that we trigger less rebuilds for the risc0 tooling if we update nixpkgs and benefit from better caching
• I updated the kairos-prover to follow the risc0pkgs template
• I packaged the kairos-prover with crane
• I refactored the source filtering for kairos, making it easier to use and likely willl improve caching because it's less error prone

Further temporary optimisation:

• uncomment the import of ./kairos-prover to avoid building any risc0 related code
• removed the GHA ubuntu build
• removed the GHA x86_64-darwin build

Note that prior to merging we need to change the merge rules to not require the ubuntu-latest check.

[github-actions[bot]]

File	Coverage
All files	54%	:x:
kairos-tx/src/asn.rs	76%	:white_check_mark:
kairos-tx/src/error.rs	0%	:x:
kairos-server/src/routes/deposit.rs	88%	:white_check_mark:
kairos-server/src/routes/transfer.rs	90%	:white_check_mark:
kairos-server/src/config.rs	0%	:x:
kairos-server/src/errors.rs	12%	:x:
kairos-server/src/state.rs	90%	:white_check_mark:
kairos-server/src/utils.rs	22%	:x:
kairos-crypto/src/implementations/casper.rs	3%	:x:
kairos-test-utils/src/cctl/parsers.rs	66%	:white_check_mark:
kairos-server/src/state/transactions.rs	57%	:x:
kairos-server/src/state/trie.rs	35%	:x:
kairos-test-utils/src/cctl.rs	87%	:white_check_mark:
kairos-test-utils/src/kairos.rs	95%	:white_check_mark:
kairos-server/tests/transactions.rs	85%	:white_check_mark:
kairos-server/src/state/transactions/batch_state.rs	42%	:x:

Minimum allowed coverage is 60%

Generated by :monkey: cobertura-action against f8122c37148afd3a96c55d241381826ca0afcd43