fix: fix an issue that multi-arch images cannot be built from the project makefile

[kurokobo]

Closes #6660

This is the first time for me to send PR to this repository. Please let me know if I'm on the wrong way.
I wasn't sure if a changelog was necessary, but I wrote one anyway.

Thanks in advance 😃

Description of the change:

Fix an issue that multi-arch images cannot be built from the project Makefile, by following two changes:

• Remove test target from docker-buildx target
• Remove sed command that adds --platform=${BUILDPLATFORM} so that ${TARGETPLATFORM} is specified as --platform according to BuildKit defaults

Motivation for the change:

Checklist

If the pull request includes user-facing changes, extra documentation is required:

• Add a new changelog fragment in changelog/fragments (see changelog/fragments/00-template.yaml)
• Add or update relevant sections of the docs website in website/content/en/docs

[everettraven]

Thanks for this contribution @kurokobo ! The changes look good to me visually. Approving and merge will be pending successful CI checks

/lgtm