fix: change `Header::nonce` to `B64` #1729

Job	Run time
docs	51s
cfg-check	25s
clippy	29s
doctest	1m 1s
feature-checks	2m 10s
fmt	14s
no-std	11s
test stable --no-default-features	1m 0s
wasm-unknown	35s
wasm-wasi	1m 6s
test stable	1m 3s
test stable --all-features	1m 52s
test nightly --no-default-features	1m 12s
test nightly	1m 13s
test nightly --all-features	2m 6s
test 1.76 --no-default-features	35s
test 1.76	34s
	16m 37s

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