gitlab-ci.yml: Move build-tools-and-docs-debian to a better place

The "build-tools-and-docs-debian" job had been added in between
the "check-system-debian" and the "accepance-system-debian" jobs
and thus separates the jobs that belong together. Move it away,
to the end of the file, next to the "pages" job that depends on it.
And while we're at it, also add a proper "needs:" line to the
job so that it can be started as soon as possible instead of always
waiting for the previous stage to finish.

Message-Id: <20210311142211.1547864-2-thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>