zcash_keys: Fix a few problems with no-flags compilation. #2554

Job	Run time
Latest build on ubuntu-latest	1m 46s
Bitrot check	4m 23s
Test on ubuntu-latest	13m 34s
Build target wasm32-wasi	4m 4s
Test on ubuntu-latest with --features orchard	13m 33s
Latest build on windows-latest	4m 12s
Clippy (MSRV)	4m 30s
Test on ubuntu-latest with --features unstable-nu6	13m 32s
Clippy (beta)	12s
Test on ubuntu-latest with --features zfuture	13m 1s
Latest build on macOS-latest	2m 26s
Test on windows-latest	17m 37s
Code coverage	20m 35s
Test on windows-latest with --features orchard	17m 44s
Intra-doc links	5m 3s
Test on windows-latest with --features unstable-nu6	17m 48s
Test on windows-latest with --features zfuture	17m 47s
protobuf consistency	5m 16s
Test on macOS-latest	18m 25s
UUID validity	6s
Rustfmt	14s
Test on macOS-latest with --features orchard	21m 54s
	3h 37m 42s

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