Remove cache_dir and use project_dir/.fractal_cache

[ychiucco]

ref #1934

from #1934 (comment)

How does it interact with the cache directory? => use project_folder as the cache directory as well

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EDIT: In all places where cache_dir was required, now project_dir is required.

[jluethi]

Yes

project_dir/.fractal_cache as the new cache directory

=> we can get rid of cache_directory variable

Let's do this once we've set up project_dir usage in all deployments