build: Install bash-completion relative to datadir

build: Install bash-completion relative to datadir

Since bash-completion 2.9, it was no longer possible to override
the completionsdir through prefix. [1] In 2.10, the overridability
was re-estabilished but this time through datadir variable. [2]

This should not really matter except for developers installing the project
into a custom prefix or distros using per-package prefixes like NixOS.

[1]: https://github.com/scop/bash-completion/commit/81ba2c7e7dfbaefbafa1e8615727c9612e5fb314
[2]: https://github.com/scop/bash-completion/pull/344