Examples should not use `temp_dir`

Things like database location this should be passed in on the command line. This creates a permanent file in the user's temp dir we never clean up. See:

https://github.com/bitcoindevkit/bdk/blob/f4d2a76661d465dd0640d54470b95cc7b5ece979/example-crates/wallet_electrum/src/main.rs#L18