gitlab-ci: Only keep the build logs as artifacts.

gitlab-ci: Only keep the build logs as artifacts.

On a build failure, we were tarring up the whole ccache directory,
build.ninja, build products, etc.  This was over 400MB compressed on a
recent early meson-main build failure, which fd.o then has to hang on
to for 4 weeks.  The build logs are probably the interesting part, are
potentially useful regardless ("how did CI's build flags differ from
mine?"), and are <500k uncompressed on my personal meson build.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>