fix: pin bitcoin commit hash

[wilfredallyn]

When I ran devenv shell, I got this error

devenv shell
• Building shell ...
• Using Cachix: devenv
error: hash mismatch in fixed-output derivation '/nix/store/lj8j0ic0jgrnv2f9rpl7bxq7rhm90d51-source.drv':
         specified: sha256-iPVtR06DdheYRfZ/Edm1hu3JLoXAu5obddTQ38cqljs=
            got:    sha256-OLSCaj1CAK3L29VZIW4ZB4TGD/PLQrUjbctYvtrzzyE=
error: 1 dependencies of derivation '/nix/store/pawxv8wy819mqsijwrq77g97qzmg6s6h-bitcoind-sv2.drv' failed to build
error: 1 dependencies of derivation '/nix/store/7vy8850lq9r0z8vc9895l9vmhpdim7mi-devenv-profile.drv' failed to build
error:
       error: 2 dependencies of derivation '/nix/store/w85984lrp1y2zl9160pg9hc10mz4p7hr-devenv-shell-env.drv' failed to build

The error is due to bitcoind.nix being pinned to the sv2 branch in sjors' repo. The hash changes whenever commits get added. I modified the file to pin the current commit hash. Is there a different commit that should be pinned?

[EthnTuttle]

LGTM.