Remove relative elements from NQP_HOME and PERL6_HOME

[patrickbkr]

This in turn results in the repository path specs to not contain any relative path elements. Fixes #2840

[lizmat]

Argh, looks like merging #2845 messed up this one.

Looks like this should be fixable by doing the nqp::rindex on $exec-dir, like:

nqp::substr($exec-dir, 0, nqp::rindex($exec-dir, $sep)) ~ '/share/nqp';

??

[lizmat]

Thanks!