Allow to configure alternate mirrors

[tnull]

It would be nice if we could point the crate to alternate mirrors rather than having https://bitcoincore.org/ hard-coded.

Main motivation is that we in LDK recently started seeing CI failures that seem to come from Github CI being unable to connect to https://bitcoincore.org/:

error: failed to run custom build command for `bitcoind v0.28.1`

Caused by:
  process didn't exit successfully: `/home/runner/work/rust-lightning/rust-lightning/target/debug/build/bitcoind-db625c734508c201/build-script-build` (exit status: 101)
  --- stdout
  filename:bitcoin-23.0-x86_64-linux-gnu.tar.gz version:23.0 hash:2cca490c1f2842884a3c5b0606f179f9f937177da4eadd628e3f7fd7e25d26d0
  url:https://bitcoincore.org/bin/bitcoin-core-23.0/bitcoin-23.0-x86_64-linux-gnu.tar.gz

  --- stderr
  thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Transport(Transport { kind: Io, message: None, url: Some(Url { scheme: "https", cannot_be_a_base: false, username: "", password: None, host: Some(Domain("bitcoincore.org")), port: None, path: "/bin/bitcoin-core-23.0/bitcoin-23.0-x86_64-linux-gnu.tar.gz", query: None, fragment: None }), source: Some(Custom { kind: TimedOut, error: "timed out reading response" }) })', /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/bitcoind-0.28.1/build.rs:106:43

If these issues continue to appear we'd be interested in exploring hosting our own mirror to make the bitcoind/electrsd based tests more robust.

[RCasatta]

Make sense to allow to override the download endpoint from an env var.
PR welcome, or I'll do it sometime whoever arrives first :)