ci: merge build-x86_64 & build-misc

The split doesn't really make sense anymore, and with the next commit,
the build-misc jobs would get delayed until the build-x86_64 jobs are
done, which is undesirable.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30808>

.gitlab-ci.yml

@@ -145,8 +145,7 @@ stages:
   - container
   - git-archive
   - build-for-tests
-  - build-x86_64
-  - build-misc
+  - build-only
   - code-validation
   - amd
   - amd-postmerge