[ci] Disable DOCKER_BUILDKIT in pipeline build for OOM issue. (#13702)

Why I did it
New docker release v23.0 uses BUILDKIT by default.
It leads to OOM issue in pipeline build.

##[error]Exit code 137 returned from process: file name '/agent/externals/node16/bin/node',
How I did it
Disable BUILDKIT when building sonic-slave-* image.
Keep checking if there are issues when building docker image inside sonic-slave-*.

How to verify it
Check docker build logs.
Disable BUILDKIT log:

Step 1/80 : FROM publicmirror.azurecr.io/debian:buster
 ---> ff5db168d4c5

.azure-pipelines/template-variables.yml

@@ -4,3 +4,4 @@ variables:
   SONIC_SLAVE_DOCKER_DRIVER: 'overlay2'
   SONIC_BUILD_RETRY_COUNT: 3
   SONIC_BUILD_RETRY_INTERVAL: 600
+  DOCKER_BUILDKIT: 0