Don't post CI failure in #stackstorm Slack for e2e tests in st2 PRs

[arm4b]

When triggering e2e tests for PRs we shouldn't post an ERROR in #stackstorm channel if they failed.

That's because we have:

st2ci/actions/workflows/st2_pkg_e2e_test.yaml

Lines 44 to 46 in 3fa73bc

	notify_failure_channels:
	- "#thunderdome"
	- "#stackstorm"

Instead to #stackstorm Slack channel should go only important build messages related to promotion and triggered on schedule.