[GO] fix generated variable names when no discriminator is used with 'oneOf'

[sazapp-spectra]

Fixes #18922. In particular, useOneOfDiscriminatorLookup needs to be true when generating to reproduce the issue; the goal is for the output to match whether it's true or false.

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@antihax @grokify @kemokemo @jirikuncar @ph4r5h4d @lwj5

[wing328]

what about adding this new schema/model to modules/openapi-generator/src/test/resources/3_0/go/petstore-with-fake-endpoints-models-for-testing-with-http-signature.yaml (the existing go test spec) instead and then regenerate the samples?

[sazapp-spectra]

done - ./bin/generate-samples.sh ./bin/configs/go-petstore.yaml -- -p useOneOfDiscriminatorLookup=true now reproduces the issue in model_one_of_with_complex_type.go (when the mustache changes are reverted).

[wing328]

looks good. let's give it a try

thanks for the PR