gitlab-ci: Actually capture Meson logs

Correct the path to the build directory so we can capture Meson logs;
especially useful when tests fail like in #184.

An example of this change having been run with a deliberately-failing
test, capturing the Meson logs, can be found at:
https://gitlab.freedesktop.org/daniels/weston/-/jobs/94623



Signed-off-by: Daniel Stone <daniels@collabora.com>