deprecate install input

[crazy-max]

follow-up:

• https://github.com/docker/buildx/pull/3472
• https://github.com/docker/docs/pull/23572

Buildx install command is deprecated since v0.30.0: https://github.com/docker/buildx/releases/tag/v0.30.0

People don't need to use it if they are using our build-push-action or bake-action. If they are not using our action and use docker build command instead, then they need to specify the BUILDX_BUILDER env var: https://docs.docker.com/build/builders/#difference-between-docker-build-and-docker-buildx-build

      -
        name: Set up Docker Buildx
        id: builder
        uses: docker/setup-buildx-action@v3
      -
        name: Build
        run: |
          docker build .
        env:
          BUILDX_BUILDER: ${{ steps.builder.outputs.name }}

Or they can directly invoke docker buildx build command so they don't need to specify this env var.