buildx: remove experimental flags

Tetsing this, I can't determine exactly when this might have switched.
It's not listed in.  Let's see...

[1] https://github.com/docker/cli/blob/master/experimental/README.md

Change-Id: If6c2ae98ea42c505f46563ca84a870081c4cdbe9

roles/build-container-image/tasks/buildx.yaml

@@ -46,15 +46,11 @@
   command: "{{ docker_buildx_command }} --platform={{ zj_image.arch | join(',') }}"
   args:
     chdir: "{{ zuul_work_dir }}/{{ zj_image.context }}"
-  environment:
-    DOCKER_CLI_EXPERIMENTAL: enabled
 
 - name: Push arch-specific layers one at a time
   command: "{{ docker_buildx_command }} --platform={{ zj_arch }} --push"
   args:
     chdir: "{{ zuul_work_dir }}/{{ zj_image.context }}"
-  environment:
-    DOCKER_CLI_EXPERIMENTAL: enabled
   loop: '{{ zj_image.arch }}'
   loop_control:
     loop_var: zj_arch
@@ -63,8 +59,6 @@
   command: "{{ docker_buildx_command }} --platform={{ zj_image.arch | join(',') }} --push"
   args:
     chdir: "{{ zuul_work_dir }}/{{ zj_image.context }}"
-  environment:
-    DOCKER_CLI_EXPERIMENTAL: enabled
 
 - name: Pull images from temporary registry
   command: >-

roles/build-container-image/tasks/setup-buildx.yaml

@@ -1,22 +1,14 @@
 - name: Update qemu-static container settings
   command: docker run --rm --privileged multiarch/qemu-user-static --reset -p yes
-  environment:
-    DOCKER_CLI_EXPERIMENTAL: enabled
 
 - name: Create builder
   command: "docker buildx create --name mybuilder --driver-opt network=host{% if buildset_registry is defined %} --config /etc/buildkit/buildkitd.toml {% endif %}"
-  environment:
-    DOCKER_CLI_EXPERIMENTAL: enabled
 
 - name: Use builder
   command: docker buildx use mybuilder
-  environment:
-    DOCKER_CLI_EXPERIMENTAL: enabled
 
 - name: Bootstrap builder
   command: docker buildx inspect --bootstrap
-  environment:
-    DOCKER_CLI_EXPERIMENTAL: enabled
 
 - name: Make tempfile for registry TLS certificate
   tempfile: