ci: pin `once_cell` to `1.20.3` for MSRV #6968

Job	Run time
prepare	4s
Check WASM	1m 1s
Build & Test Examples (example_cli)	31s
Check no_std	38s
Build & Test Examples (example_bitcoind_rpc_polling)	35s
Build & Test (1.84.1, true, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	2m 13s
Build & Test Examples (example_electrum)	50s
Rust clippy	1m 10s
Build & Test (1.84.1, true, --all-features)	3m 57s
Build & Test Examples (example_esplora)	55s
Build & Test (1.63.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	3m 30s
Rust fmt	11s
Build & Test Examples (example_wallet_electrum)	53s
Build & Test (1.63.0, --all-features)	4m 30s
Build & Test Examples (example_wallet_esplora_async)	56s
Build & Test (1.75.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	2m 18s
Build & Test Examples (example_wallet_esplora_blocking)	58s
Build & Test (1.75.0, --all-features)	4m 8s
Build & Test Examples (example_wallet_rpc)	36s
	29m 54s

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