Changes

publish-manuals-for-tags: fix build deps install

Use the absolute path when checking for debian/control in
build_publish_manuals(), as we aren't in the /build directory at that
point.

Fixes: 072b1eae88 ("manuals: publish pyosmocom manuals for releases")
Change-Id: I8749107b974a4a8d57da868829a8f548404f3fa3