test-release.sh: tweak RPATH for the binary packages.

libtool sets RPATH to "$ORIGIN/../lib:/the/directory/where/it/was/built/lib" so that a developper can use the built or the installed version seamlessly. Our binary packages should not have this developer friendly tweak, as the users of the binaries will not have the build tree.

Beside, in case the development tree is a possibly on an automounted share, this can create very bad user experience : they will incur an automount timeout penalty and will get a very bad feeling of llvm/clang's speed.