Fix platform parsing to support multiple equals signs

[TheRobotCarlson]

Use strings.LastIndex instead of strings.Split to correctly parse platform strings containing multiple equals signs (e.g., runner=fast-disk-network=catthehacker/ubuntu:act-22.04).

This allows the platform key to contain equals signs, splitting only on the last occurrence to separate the key from the value.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.74%. Comparing base (bd4bc99) to head (2f8ecdf).
⚠️ Report is 27 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5961      +/-   ##
==========================================
+ Coverage   74.65%   76.74%   +2.08%     
==========================================
  Files          73       73              
  Lines       11139     9201    -1938     
==========================================
- Hits         8316     7061    -1255     
+ Misses       2186     1505     -681     
+ Partials      637      635       -2     

☔ 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.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[crohr]

Would be nice to get this added, thanks for the PR!