Introduce variable SALSA_CI_CUT_OFF_BUILD_LOG

If the build log exceeds 4 MB the job console will just display a warning and no progress nor Gitlab-CI stage information is visible. Using this variable projects that have a very long build log will get the build log part cut off at 10000 lines and continue to display the rest of what Gitlab-CI outputs (artifacts info, cache upload status etc).

You can see the problem here: https://salsa.debian.org/mariadb-team/mariadb-10.3/-/jobs/239416

And with this fix introduced it looks good: https://salsa.debian.org/mariadb-team/mariadb-10.3/-/jobs/240436