(maint) Attempt to prevent invalid cross link device errors

When adding new cows and provisioning new builders, it is common to have a cow
build fail when trying to link/mount proc elements. The pbuilder documentation
suggests adding the APTCACHEHARDLINK=no declaration to pbuilderrc to help
minimize this possibility. I have no idea if this will actually help.

http://www.netfort.gr.jp/~dancer/software/pbuilder-doc/pbuilder-doc.html#lninvalidcrossdevicelink