Fix custom_sensor_system example

[azeey]

🦟 Bug fix

Summary

The custom_sensor_system example and CI test broke when gz-sensors' main branch was bumped to gz-sensors9. This fixes by using the gz-sensors8 branch.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #2208 (0e1170a) into gz-sim8 (8c2fb9b) will not change coverage.
The diff coverage is n/a.

❗ Current head 0e1170a differs from pull request most recent head 9e62043. Consider uploading reports for the commit 9e62043 to get more accurate results

@@           Coverage Diff            @@
##           gz-sim8    #2208   +/-   ##
========================================
  Coverage    65.92%   65.92%           
========================================
  Files          323      323           
  Lines        30719    30719           
========================================
  Hits         20251    20251           
  Misses       10468    10468