Update bitcoind.nix for NixOS

[wilfredallyn]

Running devenv up on NixOS gives this error:

Could not start dynamically linked executable: bitcoind
NixOS cannot run dynamically linked executables intended for generic
linux environments out of the box. For more information, see:
https://nix.dev/permalink/stub-ld

This PR updates bitcoind.nix so it works on NixOS

It should be confirmed that it works on mac and linux before merging.

[vnprc]

Works on linux! Need to test on mac but I would be very surprised if there are any issues.

[vnprc]

Works on mac!