Set GZ_IP for gz_src_TEST (backport #723)

[mergify]

🦟 Bug fix

Similar to #595 but for UNIT_gz_src_TEST

Summary

I noticed that UNIT_gz_src_TEST has been failing with networking error messages, so this applies the GZ_IP=127.0.0.1 fix from #595 to that test as well.

Example failures:

• https://build.osrfoundation.org/job/gz_transport-ci-gz-transport14-homebrew-arm64/5/testReport/junit/(root)/UNIT_gz_src_TEST/test_ran/
• https://build.osrfoundation.org/job/gz_transport-ci-gz-transport13-homebrew-arm64/4/testReport/junit/(root)/gzTest/cmdTopicInfo/

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)
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This is an automatic backport of pull request #723 done by [Mergify](https://mergify.com).