[BE] Migrate Anaconda Prune jobs from CircleCI to GHA (#93876)
We need periodical anaconda prune jobs to remove older packages (e.g. pytorch, torchvision, torchaudio, torchtext etc) from channels like pytorch-nightly and pytorch-test.
Currently it is done in circleci (e.g. https://app.circleci.com/pipelines/github/pytorch/pytorch/647201/workflows/72e5af30-0d54-44c1-8d9b-4c5502d27c9d/jobs/17260775) and triggered by postnightly update (https://github.com/pytorch/pytorch/tree/postnightly)
However, this postnightly branch triggers so many useless jobs (dozens of them failed due to docker command too long. Why? Because change history was part of docker command and it exceeds max INT).
<img width="756" alt="image" src="https://user-images.githubusercontent.com/109318740/216139179-3c913094-82cb-4605-99b7-23a21b4cbb36.png">
Therefore, we should stop the postnightly jobs (waste of resources) but save anaconda prune jobs.
This PR attempts to achieve this.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/93876
Approved by: https://github.com/atalman
