GHA: Windows runners don't allow privileged containers

[amargherio]

When running the build job for the Windows container images, we're using docker/setup-buildx-action@v3 to install buildx and generate a builder for use with the multi-arch builds.

As seen in the output of the triggered build test https://github.com/amargherio/mechanic/actions/runs/11407647805/job/31744276475, the action is attempting to use a privileged container. This is a known issue based on BuildKit support for Windows.

Experimental support for BuildKit on Windows was released back in March 2024 (moby/buildkit#616 (comment)) and there's documented steps for getting BuildKit running on Windows.

In order to get Windows builds functioning, we should...

• Remove the docker/setup-buildx-action call and replace it with manual steps to setup BuildKit/buildx found at https://github.com/moby/buildkit/blob/master/docs/windows.md.
• Open an issue for tracking the setup-buildx-action updates and effort to reimplement.

[amargherio]

Closing this for now. Crunching out some numbers, arm64 support is being de-prioritized and will be addressed post 1.0.0