[ci] Add variable group MAUI

[rmarinho]

Description of Change

We need a couple of variables from this variable group.
Change other variables to control internal vs public builds.
Small cleanup

This pull request updates build and pipeline configuration files to simplify variable handling, improve conditional logic for build stages, and clean up unused parameters. The primary focus is on streamlining how build arguments and variable groups are set for different build scenarios, including official, public, and internal builds.

Build and variable configuration simplification

• Removed the _InternalBuildArgs variable and its assignment from eng/pipelines/common/variables.yml, and updated how the MAUI variable group is included to always be present, rather than conditionally. [1] [2]
• Changed the logic for setting _PublishArgs in eng/pipelines/arcade/variables.yml to use build definition names instead of checking for public/internal builds and pull requests, making the conditions more explicit and maintainable.
• Set _OfficialBuildIdArgs to an empty value instead of passing parameters, simplifying the official build argument handling.

Pipeline cleanup and conditional logic improvements

• Removed the unused UseProvisionator parameter and related provisioning logic from eng/pipelines/ci-uitests.yml, reducing complexity and potential confusion in test pipeline configuration. [1] [2]

Minor fix

• Updated the Creator property condition in eng/helix.proj to check for an existing value before setting a default, preventing unwanted overrides.

[Copilot]

Pull Request Overview

This PR restructures the variable group declarations in the pipeline configuration to make the MAUI variable group unconditionally available instead of being conditionally included based on build definition names.

Key Changes

• The MAUI variable group is moved from conditional inclusion to unconditional inclusion at the top level