ci: skip Docker Hub login when credentials unavailable

[skl]

Summary

Skip Docker Hub login in test workflows when credentials are not available. This prevents workflows from failing on fork PRs and Dependabot/Renovate PRs while preserving rate limit avoidance when credentials are present.

Changes

• Added conditional to 5 test workflows: if: secrets.DOCKER_TOKEN_EBPF_INSTRUMENTATION != ''
• Affects: pull_request_integration_tests, pull_request_integration_tests_arm, pull_request_k8s_integration_tests, pull_request_oats_test, workflow_integration_tests_vm
• Docker Hub login is only needed for pull rate limits in tests, so gracefully skipping when unavailable is safe

Checklist

• Changes tested to prevent workflow failures on forks
• If this enhances / fixes / changes a core feature, I have updated the features documentation

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: skl / name: Stephen Lang (0e0e0e4)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.63%. Comparing base (0c90307) to head (0e0e0e4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1439      +/-   ##
==========================================
+ Coverage   43.74%   44.63%   +0.89%     
==========================================
  Files         312      311       -1     
  Lines       33911    28681    -5230     
==========================================
- Hits        14834    12803    -2031     
+ Misses      18120    15103    -3017     
+ Partials      957      775     -182     

Flag	Coverage Δ
integration-test	?
integration-test-arm	?
integration-test-vm-x86_64-5.15.152	?
integration-test-vm-x86_64-6.10.6	?
k8s-integration-test	?
oats-test	?
unittests	44.63% <ø> (ø)

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