CI: Add caching #4866

Job	Run time
Bitrot check	4m 35s
Build target wasm32-wasi	4m 17s
Clippy (MSRV)	4m 51s
Latest build on ubuntu-latest	52s
Clippy (beta)	13s
Latest build on windows-latest	3m 48s
Code coverage	21m 25s
Latest build on macOS-latest	1m 59s
Intra-doc links	5m 20s
Rustfmt	11s
protobuf consistency	4m 28s
UUID validity	2s
Test on ubuntu-latest-8cores	6m 3s
Test on ubuntu-latest-8cores with --features orchard	10m 7s
Test on ubuntu-latest-8cores with --features unstable-nu6	6m 11s
Test on ubuntu-latest-8cores with --features zfuture	6m 30s
Test on windows-latest-8cores	9m 6s
Test on windows-latest-8cores with --features orchard	14m 39s
Test on windows-latest-8cores with --features unstable-nu6	10m 21s
Test on windows-latest-8cores with --features zfuture	10m 48s
Test on macOS-latest	16m 45s
Test on macOS-latest with --features orchard	19m 20s
	2h 41m 51s

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