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 <email@example.com>