Refactor bitcoind.nix to fetch pre-built binaries based on platform d…

[average-gary]

…etection and update devenv.nix to configure bitcoind with a testnet configuration. This improves cross-platform compatibility and simplifies the build process.

Needs testing on a machine that is not mine.

[vnprc]

why use bash to update bitcoin.conf? You can edit it directly at /bitcoind.conf.

[vnprc]

It builds! Incredible! \o/

Getting this on jd-server startup, tho:

2025-05-01T13:25:16.136523Z  INFO jd_server::lib::job_declarator: JD INITIALIZED
2025-05-01T13:25:16.392614Z ERROR jd_server::lib::mempool::error: Rpc(Deserialization("EOF while parsing a value at line 1 column 0"))
2025-05-01T13:25:16.392623Z ERROR jd_server::lib::mempool::error: Unable to establish RPC connection with Template Provider (possible reasons: not fully synced, down)
2025-05-01T13:25:16.392645Z ERROR jd_server::lib: SHUTDOWN from Upstream: Mempool error: `Rpc(Deserialization("EOF while parsing a value at line 1 column 0"))`
Try to reconnecting or connecting to a new upstream

I think you have clobbered the username/pw that was previously injected using bitcoind.conf. Will attempt to repair.

[vnprc]

Fixed it in this commit: a52584b

i dunno how to get that commit in this PR but fuck it, merging to master #yolo