fix(electrum): prevent `fetch_prev_txout` from querying coinbase transactions #6453

Job	Run time
Check no_std	9s
Check WASM	25s
Build and test (stable, true, --no-default-features --features miniscript/no-std)	1m 31s
Rust fmt	5s
Build and test (stable, true, --all-features)	3m 45s
prepare	3s
Build and test (1.63.0, --no-default-features --features miniscript/no-std)	2m 7s
Build and test (1.63.0, --all-features)	4m 58s
Build & Test Examples (example_cli)	22s
Build & Test Examples (example_bitcoind_rpc_polling)	15s
Build & Test Examples (example_electrum)	40s
Build & Test Examples (example_esplora)	46s
Build & Test Examples (example_wallet_electrum)	46s
Build & Test Examples (example_wallet_esplora_async)	55s
Build & Test Examples (example_wallet_esplora_blocking)	46s
Build & Test Examples (example_wallet_rpc)	27s
clippy_check	35s
	18m 35s

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