Make the file_cache_path directory configurable

When the cookbook is running as root, chef sets the chef directory (top
directory of cache folder) to 700. When the homebrew cookbook is
executed the homebrew_go script is downloaded to chef cache. The file
can't be executed by the non root owner because the chef top level
directory is not readable by the user:

$ sudo ls -ld chef chef/cache chef/cache/homebrew_go
drwx------  5 root  root   170 May 28 11:53 chef
drwxr-xr-x  5 root  root   170 May 28 11:53 chef/cache
-rwxr-xr-x  1 root  root  7231 May 28 11:53 chef/cache/homebrew_go

In general it might be a better idea to download the homebrew_go script
to /tmp and only download it if homebrew is not installed. But as a
first fix this patch makes the file_cache_path configurable over an
attribute.