Fix missing CircleCI GPG key (#16961)
Summary:
I'm seeing a bunch of apt gpg key errors on CI with the following message:
```
An error occurred during the signature verification. The repository is not
updated and the previous index files will be used. GPG error:
https://packagecloud.io trusty InRelease: The following signatures couldn't
be verified because the public key is not available:
NO_PUBKEY 4E6910DFCB68C9CD
```
Most of the times apt will reuse the old cached version, but sometimes this results in a build failure: https://circleci.com/gh/pytorch/pytorch/758366?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link.
This should hopefully fix it.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16961
Differential Revision: D14028151
Pulled By: ezyang
fbshipit-source-id: 7648a0a58ece38d8d04916937a9fa17f34f8833e
