(GH-1055) Fix hardcoded cache path

(GH-1055) Fix hardcoded cache path

Prior to this commit the cache path used to create the script file resource
was hardcoded to /opt/puppetlabs/puppet/cache.

This commit fixes that by using the `puppet_vardir` fact provided by stdlib so
that we will always get the correct path for the OS that is executing the code.

Additionally, if for some reason the `puppet_vardir` fact is not available we
will fall back to `tmp`.