Nix: Upgrade nixpkgs to enable cross-compilation to RISC-V Musl

Part of RV-332

What

Upgrades nixpkgs. This also requires an update to our opam-nix-integration dependency.

Why

We need a more recent checkout of nixpkgs to simplify cross-compilation, as there have been several improvements in that area. This enables us to compile RISC-V kernels to the riscv64gc-unknown-linux-musl target effortlessly.

Manually Testing

nix develop -j auto gitlab:tezos/tezos/ole-nix-musl --command true