Add TURBO_CACHE_DIR env for `--cache-dir` flag (#6294)
### Description
Adds the ability to set the `--cache-dir`-flag via an environment
variable named `TURBO_CACHE_DIR`.
### Testing Instructions
TURBO_CACHE_DIR=.turbo-cache turbo run build
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Co-authored-by: Chris Olszewski <chris.olszewski@vercel.com>
Co-authored-by: Anthony Shew <anthony.shew@vercel.com>
