Run s390x build on push to main only

[andreasgerstmayr]

Which problem is this PR solving?

• Resolves #2468
• Resolves #2399

Description of the changes

• Run s390x build on push to main only due to the long duration of this test. See #2468 for more details.

How was this change tested?

Checklist

• [X] I have read https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
• [X] I have signed all commits
• [ ] I have added unit tests for the new functionality
• [ ] I have run lint and test steps successfully
  • for jaeger: make lint test
  • for jaeger-ui: yarn lint and yarn test

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.61%. Comparing base (35f10af) to head (e57ed57). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2481 +/- ## ======================================= Coverage 96.61% 96.61% ======================================= Files 254 254 Lines 7679 7679 Branches 1997 1931 -66 ======================================= Hits 7419 7419 Misses 260 260 ```

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