(Re-)Apply cache in CI builds

[Kailai-Wang]

Context

We have been suffering from the long building cycle of parachain/enclave build in CI - even with a minute change in source code we need to go through the whole compilation process.

We had a few attempts to accelerate the whole process but they weren't so successful:

• Accelerate the rust compilation #259
• Remove cargo-chef and buildx cache #659
• Sccache doesn't seem to work in CI #935

But things might have changed and a retry is worth it. Ideas:

• use buildx + cache mounts (with sccache) with docker, or
• use plain sccache with binary build

Priority is set to high because it affects (benefits) all upcoming builds

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[Kailai-Wang]

buildx cache + cache mount didn't work - the problem is the cache mount isn't exported as image layer, so it won't get cached between CI runs.

See:moby/buildkit#1512

Here's an informative summary of the problem and its background, including possible solutions(workarounds): https://hackmd.io/@kobzol/S17NS71bh

[Kailai-Wang]

a plain copy in/out of sccache data seems to work - will submit a PR soon.