Merge bitcoindevkit/bdk#1722: fix(testenv): disable downloads (bitcoind and electrsd...

9b48fd43fe203ee6dc7451264768093d9cc61297 fix(testenv): disable downloads (bitcoind and electrsd) for docs.rs builds of crate testenv (River Kanies)

Pull request description:

  …uilds of crate testenv

  ### Description

  address this issue https://github.com/bitcoindevkit/bdk/issues/1627
  where docs.rs build is failing for bdk_testenv crate

  original PR: https://github.com/bitcoindevkit/bdk/pull/1679

  ### Notes to the reviewers

  I was not able to reproduce the build issue locally, so this will have to be tested live unless someone else can reproduce the build error https://docs.rs/crate/bdk_testenv/0.10.0/builds/1377651

  more details in this thread on discord https://discord.com/channels/753336465005608961/1265333904324427849/1304476756660719668

  #### Bugfixes:

  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  ValuedMammal:
    ACK 9b48fd43fe203ee6dc7451264768093d9cc61297 but I guess we won't know for sure until we publish the crate
  notmandatory:
    ACK 9b48fd43fe203ee6dc7451264768093d9cc61297

Tree-SHA512: fe4ad420f71530a279e1cf86af6e7bd59f381804a5c7c9636fa2aaa5592ff4e6fed668513831d7d63ed0a97152d685b187d50e906b21dbbb2180dedf5d28fad0