build: replace "std" flag dep in "miniscript" flag #32

Job	Run time
Rust fmt	17s
prepare	5s
Rust clippy	1m 22s
Build & Test (1.88.0, true, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	2m 11s
Build & Test (1.63.0, --all-features)	6m 41s
Build & Test (1.63.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	5m 33s
Build & Test (1.75.0, --all-features)	4m 17s
Build & Test Examples (example_bitcoind_rpc_polling)	49s
Check no_std	45s
Check WASM	1m 11s
Build & Test (1.75.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	2m 36s
Build & Test (1.88.0, true, --all-features)	4m 1s
Build & Test Examples (example_esplora)	1m 1s
Build & Test Examples (example_cli)	1m 3s
Build & Test Examples (example_electrum)	1m 3s
	32m 55s

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