Revert "stop using an absolute rpath"

This reverts commit 708395d65d689080cc96dd091adab3fc4c5c3f27.
auto merge of #7192 : brson/rust/rpath, r=thestinger

r? @thestinger

This reverts commit 708395d65d689080cc96dd091adab3fc4c5c3f27.

Fixes #7191.