Disable gem documentation generation on Cirrus

[jmagman]

Description

Pass in -N when gem installing on Cirrus to disable document generation. This flag is already used in the Dockerfile for coveralls, bundler, etc.

Shaves off ~2.5 seconds on my machine. Not the most impressive, but there's no need for gem documents on CI so save the time, networking, and disk space.

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• Yes, this is a breaking change (Please read [Handling breaking changes]). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change.

[christopherfujino]

LGTM