fix(helm): use image.global.registry for imageExperimental

[jakubdyszkiewicz]

Summary

Instead of hardcoding registry, we leave it empty so we can infer this from image.global.registry

Issues resolved

No issues reported.

Documentation

- [ ] Link to the website documentation PR

Testing

• Unit tests
• E2E tests
• Manual testing on Universal
• Manual testing on Kubernetes

Backwards compatibility

- [ ] Update UPGRADE.md with any steps users will need to take when upgrading.
- [ ] Add backport-to-stable label if the code follows our backporting policy

[codecov-commenter]

Codecov Report

Merging #4641 (0740d39) into master (589eb10) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4641   +/-   ##
=======================================
  Coverage   46.57%   46.57%           
=======================================
  Files         684      684           
  Lines       46323    46323           
=======================================
+ Hits        21576    21577    +1     
+ Misses      22834    22833    -1     
  Partials     1913     1913           

Impacted Files	Coverage Δ
pkg/xds/cache/once/cache.go	87.17% <0.00%> (-7.70%)	⬇️
pkg/xds/generator/direct_access_proxy_generator.go	90.90% <0.00%> (+1.13%)	⬆️
pkg/plugins/leader/postgres/leader_elector.go	89.36% <0.00%> (+6.38%)	⬆️

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