Update px-sock-shop demo to use ghcr.io mirrored images

[ddelnano]

Summary: Update px-sock-shop demo to use ghcr.io mirrored images

Relevant Issues: Fixes the sock shop issues of #1905

Type of change: /kind infra

Test Plan: Deployed the demo by building and serving the px-sock-shop tar.gz locally

• Verified amqp, redis, mysql and http data was visible

$ bazel build //demos:px-sock-shop
$ cp bazel-bin/demos/px-sock-shop.tar.gz demos/ && cd demos && python -m http.server
$ px demo deploy  --artifacts http://localhost:8000 px-sock-shop

Changelog Message: Moved hosting of px-sock-shop container images from gcr.io to ghcr.io