gitlab-ci: reduce the number of scons build

It seems overkill to me to build scons 7x for every pipeline.
Scons is now build with the oldest llvm version in scons-old-llvm
and with the newest llvm version in scons.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
39 jobs for !2830 with ci-scons-opt in 52 minutes and 56 seconds (queued for 15 seconds)
merge request