CI: tweak OCI workflow trigger (backport #13890)

[mergify]

Proposed Changes

Building on push to any branch is wasteful and unnecessary, because most
of built images are never used. The workflow dispatch trigger covers the
use case to build an image from the latest commit in a branch.

The use case to validate/QA a PR is now covered by on pull request
trigger. This trigger has a caveat: PRs from forks won't produce a
docker image.

Why?
Because PRs from forks do not inject rabbitmq-server secrets. This is a
security mechanism from GitHub, to protect repository secrets.

With this trigger is possible to QA/validate PRs from other Core team
members. Technically, anyone with 'write' access to our repo to push
branches.

Types of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Checklist

Put an x in the boxes that apply.
You can also fill these out after creating the PR.
If you're unsure about any of them, don't hesitate to ask on the mailing list.
We're here to help!
This is simply a reminder of what we are going to look for before merging your code.

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• If relevant, I have added necessary documentation to https://github.com/rabbitmq/rabbitmq-website
• If relevant, I have added this change to the first version(s) in release-notes that I expect to introduce it

---

This is an automatic backport of pull request #13890 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 4efb3df has failed:

On branch mergify/bp/v4.1.x/pr-13890
Your branch is up to date with 'origin/v4.1.x'.

You are currently cherry-picking commit 4efb3df39.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .github/workflows/oci-make.yaml

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally