In 6c16ee0feead3f01c575af2b998fe614c207ac96 the environment variable was used
while reading from `nx.json`. But in `packages/workspace/src/tasks-runner/cache.ts`
the `cacheDirectory` is used without using `readCacheDirectoryProperty`. Now we
check the environment variable in `cacheDirectory`, so that the environment
variable is always used.
ISSUES CLOSED: #6629
Use the environment variable NX_CACHE_DIRECTORY to set the cache directory.
This would override the cache directory set in nx.json.
ISSUES CLOSED: #6629