ci: use idempotent labels when creating a release candidate pull request

[zimeg]

Changelog

N/A - We're testing RC in "production" it seems...

Summary

This PR follows #558 to use idempotent labels when creating a release candidate pull request instead of attempting to add labels to a new request.

Preview

Error: https://github.com/slackapi/slack-cli/actions/runs/26968804619/job/79578379382

unknown flag: --add-label

Testing

Sharing here with findings 🫡

Notes

The "remove" tag and "add" tag in updated PRs causes clutter but might be separate to follow up on I think!

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[zimeg]

🚢 Fast merge to follow up as needed!

[zimeg]

🥂 Success: #579

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.67%. Comparing base (8bd01fe) to head (4474887).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #578      +/-   ##
==========================================
+ Coverage   71.66%   71.67%   +0.01%     
==========================================
  Files         226      226              
  Lines       19148    19148              
==========================================
+ Hits        13723    13725       +2     
- Misses       4216     4217       +1     
+ Partials     1209     1206       -3     

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