Use pip3 instead of pip when building ECR GC image (#58334)
Summary:
A followup to https://github.com/pytorch/pytorch/issues/58309, to fix the broken docker_for_ecr_gc_build_job:
- https://app.circleci.com/pipelines/github/pytorch/pytorch/322672/workflows/4877ddfe-eee1-4116-91ae-6ee9dd3a97ad/jobs/13486207
- https://app.circleci.com/pipelines/github/pytorch/pytorch/322710/workflows/8d33afb6-7b85-48c7-94fd-ac9176f4a16e/jobs/13488388
- https://app.circleci.com/pipelines/github/pytorch/pytorch/322759/workflows/b480989a-b39e-48f7-929d-66f1bdc50c89/jobs/13490919
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58334
Test Plan:
Before this PR, this fails:
```
cd .circleci/ecr_gc_docker && docker build .
```
After this PR, it succeeds.
Reviewed By: zhouzhuojie
Differential Revision: D28457290
Pulled By: samestep
fbshipit-source-id: d8d8c8759412d9d7876d5908768ee5cb7261132d
