The helixbot user does not need to be set as the default user

The Helix Dockerfiles should be updated to not set `helixbot` as the default user. They should be left with `root` as the default. This works because the Helix infrastructure will explicitly set `helixbot` as the user when running the container. And having it set to `root` by default will enable the container image to be used as a container job (see https://github.com/dotnet/dotnet-buildtools-prereqs-docker/pull/1276#issuecomment-2605791196).

Find the Helix Dockerfiles and inspect how their default container user is set. Update the images so root remains the default while Helix infrastructure can still select helixbot explicitly; verify the affected Dockerfiles no longer default to helixbot.

Written by the indexing model from the issue text.