feat(pkgs/risc0): Add unstable feature flag

[Syndamia]

This adds support for certain unstable precompiles

[github-actions]

Thanks for your Pull Request!

Below you will find a summary of the cachix status of each package, for each supported platform.

package	x86_64-linux	x86_64-darwin	aarch64-darwin
avalanche-cli	✅ cached	✅ cached	✅ cached
blst	✅ cached	✅ cached	✅ cached
bnb-beacon-node	✅ cached	✅ cached	✅ cached
cardano	✅ cached	✅ cached	🚫 not supported
cdt	✅ cached	🚫 not supported	🚫 not supported
circom	✅ cached	✅ cached	✅ cached
circom_runtime	✅ cached	✅ cached	✅ cached
corepack-shims	✅ cached	✅ cached	✅ cached
cosmos-theta-testnet	✅ cached	✅ cached	✅ cached
emscripten	✅ cached	✅ cached	✅ cached
eos-vm	✅ cached	🚫 not supported	🚫 not supported
ffiasm	✅ cached	✅ cached	🚫 not supported
ffiasm-src	✅ cached	✅ cached	🚫 not supported
gaiad	✅ cached	✅ cached	✅ cached
graphql	✅ cached	✅ cached	🚫 not supported
jolt	✅ cached	🚫 not supported	🚫 not supported
jolt-guest-rust	✅ cached	🚫 not supported	🚫 not supported
kurtosis	✅ cached	🚫 not supported	🚫 not supported
nexus	✅ cached	🚫 not supported	🚫 not supported
pistache	✅ cached	🚫 not supported	🚫 not supported
polkadot	✅ cached	✅ cached	✅ cached
polkadot-fast	✅ cached	✅ cached	✅ cached
rapidsnark	✅ cached	✅ cached	🚫 not supported
rapidsnark-gpu	✅ cached	🚫 not supported	🚫 not supported
rapidsnark-server	✅ cached	🚫 not supported	🚫 not supported
risc0	✅ cached	🚫 not supported	🚫 not supported
risc0-rust	✅ cached	🚫 not supported	🚫 not supported
sp1	✅ cached	🚫 not supported	🚫 not supported
sp1-rust	✅ cached	🚫 not supported	🚫 not supported
wasmd	✅ cached	🚫 not supported	🚫 not supported
zkm	✅ cached	🚫 not supported	🚫 not supported
zkm-rust	✅ cached	🚫 not supported	🚫 not supported
zkwasm	✅ cached	🚫 not supported	🚫 not supported
zqfield-bn254	✅ cached	✅ cached	🚫 not supported