fix: respect DENO_DIR and XDG_* environment variables for fs cache

Closes #357. To resolve the "configDir" (now appropriately renamed to "cacheDir"); 1. `$DENO_DIR` is checked, and is the result if present. 2. Then `$XDG_CACHE_HOME` is checked, if present the result becomes `$XDG_CACHE_HOME/deno`. 4. Finally defaults to `$HOME/.deno` if the above two fail. The path resolved from the above three steps is then joined with the `deployctl` subdirectory.