MT#7979 jjb: keep only the recent 12 builds

20 builds are quite heavy for our matrix jobs:

[...]
5.9G    kamailio40-nightly-repos
6.0G    kamailio40-nightly-binaries
6.6G    kamailio41-nightly-binaries
6.6G    kamailio41-nightly-repos
6.8G    kamailiodev-nightly-repos
7.0G    kamailiodev-nightly-binaries

So reduce fs usage by keeping only 12 builds.
MT#7979 Do not keep binary files in *-repos jobs

Our matrix jobs are quite big (output from fs usage
with 20 recent builds being kept):

5.9G    kamailio40-nightly-repos
6.0G    kamailio40-nightly-binaries
6.6G    kamailio41-nightly-binaries
6.6G    kamailio41-nightly-repos
6.8G    kamailiodev-nightly-repos
7.0G    kamailiodev-nightly-binaries

There shouldn't be any need to keep the binary files
(gz/bz2/xz/deb) in the *-repos jobs, because the repos
jobs don't have any further downstream jobs and the
artifacts are stored within the *-binaries jobs anyway,