[explorer] Fix address conversion on testnet

Summary: When visiting an address page on the testnet explorer, the legacy address doesn't use the testnet network. This diff fixes this bug.

Test Plan:

cd web/explorer/explorer-exe
cp config.dist.toml config.toml

Edit config.toml to add:

chain = "testnet"

And change the chronik server to chronik-testnet.fabien.cash.
Then:

cargo run

Open a address page and check the legacy address is correct.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17869