Allow turning off Slack notifications for Argo Workflows

[xnyo]

Especially useful for provisioned plugins using the CD approach who don't want to spam Slack with dev deployments for each commit

We have an input here:

plugin-ci-workflows/.github/workflows/cd.yml

Line 244 in bd860d0

argo-workflow-slack-channel:

argo-workflow-slack-channel: ${{ (github.event_name == 'push' && github.ref_name == 'main') && '' || '#grafana-plugins-platform-ci' }}

But the Argo Workflow may not handle it properly