Move fs cache location. (#8126)
### Description
Moves the file system cache to `.turbo` as discussed in #842.
### Testing Instructions
Find your favorite Turborepo of choice and run a `turbo` command twice.
You should miss cache on the first run and hit cache on the second.
CLOSES TURBO-3130
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Co-authored-by: Chris Olszewski <chris.olszewski@vercel.com>
