Bump images for 2.9 release branch

[sachindshinde]

This PR:

• Bumps the Ubuntu image to the same used in Router (Router did the bump to avoid CI brownouts).
  • We were seeing the same CI brownouts, so we're doing the same.
• Stops skipping brew update when installing cmake (Router never did this apparently).
  • This makes builds take longer, but without it we're seeing brew errors when installing cmake.