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>

.gitlab-ci.yml

@@ -61,8 +61,11 @@ debian:
     paths:
       - ccache
   artifacts:
-    when: on_failure
-    untracked: true
+    when: always
+    paths:
+      - _build/meson-logs/*.txt
+      # scons:
+      - _build/*/config.log
   variables:
     CCACHE_COMPILERCHECK: "content"
   # Use ccache transparently, and print stats before/after