[FEATURE] Parallel cache merging

[deemp]

Description of a problem the feature solves

Currently, cache merging is sequential.
Therefore, the next cache is downloaded only after the former cache is unpacked.
However, the next cache can be downloaded concurrently when the former cache is downloaded or when the former cache is being unpacked.

Describe the solution you'd like

Restore multiple caches concurrently.
Possibly, it's safe, at least in the case of the Nix store where paths are not overwritten if already present.

Describe alternatives you've considered

Write custom logic for separately downloading and unpacking caches.