[CI] Bump GitHub Actions Ubuntu machines to ubuntu-latest

Summary:
We were previously setting this to 18.04, but 20.04 is the latest stable release of Ubuntu. I figure we can just go with ubuntu-latest (which is currently an alias for 20.04) to avoid having to specify a version number. Open to specifying 20.04 instead.. or any other possible approaches.

(Addresses note from @ashoat at: https://phab.comm.dev/D4239?id=13406#inline-25982)

Test Plan: Will land this and see what happens. Could fork the repo and populate the "Secrets" and whatnot to test this beforehand, but this diff is easy enough to revert if something goes wrong.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: Adrian, yayabosh, ashoat

Differential Revision: https://phab.comm.dev/D4261