extension-tool/build: Install completions relative to our datadír

It is a good practice to install files relative to our own variables

https://www.bassi.io/articles/2018/03/15/pkg-config-and-paths/

and it is required on systems like NixOS.

Thanks to Iñigo, bash-completion support that since 2.10:

https://github.com/scop/bash-completion/pull/344