Fix k8s entrypoint management for build container

What does this MR do?

Revert "Merge branch 'k8s-revert-entrypoint-changes' into 'main'"

This reverts commit 3d8a062f, reversing changes made to 1ba8b5d3.

We announced in this issue #37277 that the feature will be re added as a breaking change. However, as the breaking change issue wasn't created on time, the feature is added behind a feature flag. A follow up deprecation and breaking change issues will be created afterward.

This MR introduces FF_USE_BUILD_IMAGE_ENTRYPOINT which will be removed in %18.0 (Issue will be created when the MR is merged)

Why was this MR needed?

Allow users to use the gitlab-ci image entrypoint and command for the executorkubernetes

What's the best way to test this MR?

Passing integration test:

go test -timeout=60m -tags=integration,kubernetes -run TestEntrypointNotIgnored -v gitlab.com/gitlab-org/gitlab-runner/executors/kubernetes

What are the relevant issue numbers?

close #37277