Disable unnecessary jobs in pipeline when merging to master
As discussed in #5983 (comment 267627509) we currently run the code_quality and dependency_scanning jobs for both MRs and merges to master.
Since no direct commits to master is allowed and MRs can't be merged until the pipeline passes it is sufficient to only run these two jobs on the MR pipeline.
This will improve pipeline speed by eliminating jobs on master which only need to run once for each commit, and should have already been run as part of a merge request.
See also more similar follow-on work in #6190 (closed)
Edited by Chad Woolley