Add secure pr build triggers.

[HumairAK]

The issue resolved by this Pull Request:

Related ##243

Description of your changes:

GH does not allow PR workflows to access secrets (for obvious reasons). This pr proposes a secure way to execute pr image builds in an unprivileged environment. Workflows triggered via workflow_run event are always triggered from default branch, as such can access repository tickets.

Testing instructions

Tested with a fork pr here.
Triggered workflow when pr closed: 1
Triggered work when pr opened: 2

Checklist

• The commits are squashed in a cohesive manner and have meaningful messages.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work

[openshift-ci]

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[gmfrasca]

/lgtm