upgrade getdeps GitHub actions to Ubuntu 20

[chadaustin]

Summary: Sadly, even though Ubuntu 18.04 is still in LTS, GitHub is deprecating its runner image.

Migrate the generated GitHub Actions to 20.04.

https://github.com/actions/runner-images/issues/6002

https://github.blog/changelog/2022-08-09-github-actions-the-ubuntu-18-04-actions-runner-image-is-being-deprecated-and-will-be-removed-by-12-1-22/

Reviewed By: fanzeyi

Differential Revision: D38877286

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D38877286