fix: Modifying the trigger for the mutateexisting policy to the Nimbus policy

[shivaccuknox]

Modifying the trigger for the mutateexisting policy to a Nimbus policy. Since it is guaranteed
that a nimbus policy will be created before a mutateexisting policy, we can ensure that the mutateexisting
policy will always be triggered.

Description

Fixes #215

Does this PR introduce a breaking change?
No

Checklist

• [*] PR title follows the <type>: <description> convention
• [*] I use conventional commits in my commit messages
• I have updated the documentation accordingly
• [*] I Keep It Small and Simple: The smaller the PR is, the easier it is to review and have it merged
• [*] I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Additional information for reviewer

I have tested in my local setup (make run of the nimbus-kyverno adapter), and it works.

Mention if this PR is part of any design or a continuation of previous PRs