[perl] generate an Omnibus-friendly CPAN config

This will ensure CPAN can use the Omnibus cache directory for building.  Invocations of Perl will need to be updated to pass the `-j` option for this to have any effect, though.

This PR configures the CPAN home to `/var/cache/omnibus/cache/cpan`. Previously the CPAN home leaked out into `$HOME/.cpan` and could cause build slaves to run out of inodes as this directory was never cleaned up!

(@schisamo did the work; I'm just the messenger.)