Examples should not use `temp_dir`

[LLFourn]

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:

bdk/example-crates/wallet_electrum/src/main.rs

Line 18 in f4d2a76

let db = Store::<bdk::wallet::ChangeSet>::new_from_path(DB_MAGIC.as_bytes(), db_path)?;

[notmandatory]

Put this one in the beta.0 milestone since it's not a core functional change.

[ValuedMammal]

I think this can be closed now that all example crates are fixed to no longer use std::env::temp_dir