zcash_keys: Fix a few problems with no-flags compilation. #2561

Job	Run time
Bitrot check	4m 21s
Latest build on ubuntu-latest	1m 55s
Latest build on windows-latest	4m 28s
Clippy (MSRV)	4m 35s
Build target wasm32-wasi	4m 6s
Clippy (beta)	13s
Latest build on macOS-latest	4m 56s
Code coverage	20m 44s
Intra-doc links	6m 9s
protobuf consistency	4m 55s
Rustfmt	13s
UUID validity	3s
Test on ubuntu-latest	13m 20s
Test on ubuntu-latest with --features orchard	12m 54s
Test on ubuntu-latest with --features unstable-nu6	13m 4s
Test on ubuntu-latest with --features zfuture	13m 13s
Test on windows-latest	17m 55s
Test on windows-latest with --features orchard	18m 48s
Test on windows-latest with --features unstable-nu6	18m 6s
Test on windows-latest with --features zfuture	17m 31s
Test on macOS-latest	31m 22s
Test on macOS-latest with --features orchard	23m 39s
	3h 56m 30s

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