Disable doc generation in grpc gem compilation for FIPS builds

What does this MR do?

In !6172 (merged), a FIPS build reinstalls the gRPC gem with a patched version that links against the system OpenSSL.

The gem install command installs 9 MB of documentation for the gRPC gem. We don't need this documentation, so let's just disable it.

Related issues

Test pipeline: https://dev.gitlab.org/gitlab/omnibus-gitlab/-/merge_requests/222

Checklist

See Definition of done.

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Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened