Add UNSTABLE_CACHE_KEY environment variable

[muddyfish]

Description of change

Add UNSTABLE_CACHE_KEY environment variable.
Using UNSTABLE_CACHE_KEY allows users to specify a cache path disambiguator. This key can be used to ensure that cache directories are unique, even if the cache directory specified is shared.

Relevant issues: N/A

Does this change impact existing behavior?

No breaking changes. This will change behaviour when the UNSTABLE_CACHE_KEY variable is passed.

Does this change need a changelog entry in any of the crates?

No, this is being added as an unstable feature without documentation or promise for continued support.

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[passaro]

LGTM. Remaining suggestions are optional.

[passaro]

LGTM

[passaro]

FYI: this is fine, but we usually prefer using a reference in the match, like this:

        let managed_cache_path = match &cache_key {
            None => mountpoint_cache_path.clone(),
            Some(cache_key) => mountpoint_cache_path.join(hash_cache_key(cache_key.as_bytes())),
        };