mempool.conf, nginx.conf were not used in nix-bitcoin.
- Use `sourceRoot` - Allow `generate.sh` to directly trigger the build of `nodeModules`. Needed by mempool 3.2.1, where `mempool-backend` has multiple fixed output derivation dependencies, which can only be updated if they can be built directly.
Remove `v` prefix of mempool drv versions: `v2.5.0` -> `2.5.0`