Heptapod CI: run for MRs, but not for branch pushes

These don't bring much and are prone to fail or trigger the
cache corruption problem.

Only the post-landing case was somewhat interesting (but we didn't have
MR builds). We'll replace that by switching to the fast-forward merge
method: this only means that branch merges will have to be done in-topic.

branch : heptapod-stable
4 jobs for !57 with topic/heptapod-stable/mr-pipelines-no-push in 24 minutes and 30 seconds (queued for 2 seconds)
latest merge request
Name Stage Failure
pkg-ubuntu-focal Package
/builds/heptapod/omnibus-heptapod/lib/gitlab/util.rb:24:in `section'
/builds/heptapod/omnibus-heptapod/lib/gitlab/tasks/build.rake:15:in `block (2 levels) in <top (required)>'
/builds/heptapod/omnibus-heptapod/.bundle/ruby/2.7.0/gems/rake-13.0.6/exe/rake:27:in `<top (required)>'
/usr/local/bin/bundle:23:in `load'
/usr/local/bin/bundle:23:in `<main>'
Tasks: TOP => build:project
(See full trace by running task with --trace)
Cleaning up file based variables
ERROR: Job failed: exit code 1