fix: tag CEL Function return correct tag when tag and digest are present

[lukeelten]

Explanation

This PR fixes a bug is the tag CEL function which returns an empty string when a tag and a digest are present.

Related issue

Fixes kyverno/kyverno#15273

Proposed Changes

The root issue can be found upstream in google/go-containerregistry#2069 but there has been no progress since more than a year.
To fix this issue, the edge case is detected in kyverno cel function and handled properly.
The special case can be removed when the bug has been fixed in upstream.

Checklist

• I have read the contributing guidelines.
• I have read the PR documentation guide and followed the process including adding proof manifests to this PR.
• This is a bug fix and I have added unit tests that prove my fix is effective.

Further Comments

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.58%. Comparing base (b97f888) to head (76b3f1c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
+ Coverage   71.52%   71.58%   +0.05%     
==========================================
  Files          79       79              
  Lines        2381     2386       +5     
==========================================
+ Hits         1703     1708       +5     
  Misses        536      536              
  Partials      142      142              

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[realshuting]

Thanks @lukeelten for the contribution, this looks good overall!

Can you please address this semantic check issue?

[lukeelten]

Hi @realshuting ,
I updated the commit message and signed the commit.