nostr: fix `is_replaceable` method #1013

Job	Run time
Build no_std (nightly)	2m 24s
Format	7s
Build (stable, -p nostr)	2m 39s
Build (stable, -p nostr --no-default-features --features alloc)	41s
Build WASM (stable, -p nostr)	33s
Build (stable, -p nostr --no-default-features --features "alloc all-nips")	1m 59s
Build WASM (stable, -p nostr-sdk)	41s
Build (stable, -p nostr --features blocking)	2m 14s
Build WASM (stable, -p nostr-js)	32s
Build (stable, -p nostr-sdk)	2m 58s
Build WASM (1.64.0, -p nostr)	5m 13s
Build WASM (1.64.0, -p nostr-sdk)	5m 33s
Build (stable, -p nostr-sdk --no-default-features)	1m 48s
Build WASM (1.64.0, -p nostr-js)	5m 14s
Build (stable, -p nostr-sdk --features blocking)	3m 47s
Build (stable, -p nostr-ffi)	2m 18s
Build (stable, -p nostr-sdk-ffi)	3m 19s
Build (1.64.0, -p nostr)	6m 27s
Build (1.64.0, -p nostr --no-default-features --features alloc)	5m 26s
Build (1.64.0, -p nostr --no-default-features --features "alloc all-nips")	6m 9s
Build (1.64.0, -p nostr --features blocking)	6m 44s
Build (1.64.0, -p nostr-sdk)	7m 26s
Build (1.64.0, -p nostr-sdk --no-default-features)	7m 8s
Build (1.64.0, -p nostr-sdk --features blocking)	8m 5s
Build (1.64.0, -p nostr-ffi)	7m 51s
Build (1.64.0, -p nostr-sdk-ffi)	8m 22s
	1h 45m 38s

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