The update_responses script should use mar-tools from the release`s directory

The current version of the update_responses script is using the mar-tools from the path ../../../gitian-builder/inputs/mar-tools-linux64.zip.

This works fine when a build with the correct version has been done before from this directory, but this is not always the case.

Instead, it should be using the mar-tools-linux64.zip from the release`s directory.

This should also help for legacy/trac#17381 (moved).