[K8s Plugin] Add e2e tests for K8S_ROLLBACK stage and fix the bug

[Warashi]

What this PR does:

• add simple E2E test for K8S_ROLLBACK stage
• add a variant label to manifests when executing the K8S_ROLLBACK stage
  • to make consistent with the K8S_SYNC stage

Why we need it:

• to check and keep the k8s plugin's behavior as the test

Which issue(s) this PR fixes:

Part of #4980

Does this PR introduce a user-facing change?: No

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 25.75%. Comparing base (2aa01ac) to head (52cdd4a).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5683      +/-   ##
==========================================
+ Coverage   25.66%   25.75%   +0.08%     
==========================================
  Files         476      476              
  Lines       50894    50900       +6     
==========================================
+ Hits        13063    13109      +46     
+ Misses      36841    36794      -47     
- Partials      990      997       +7     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[t-kikuc]

LGTM

If needed in the future, let's find a way to split simple unit tests and e2e tests for faster testing.

[ffjlabo]

LGTM