Run s390x build on push to main only

[andreasgerstmayr]

Which problem is this PR solving?

• Resolves Need to speed up s390x-build #2468
• Resolves Build on s390x / s390x-build (pull_request) takes forever #2399

Description of the changes

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

How was this change tested?

Checklist

• I have read https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
• 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@           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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