Ensure custom feature gate mainfest path is passed into generator

[JoelSpeed]

Without passing this field into the new generator options, the flag is redundant. That's ok for o/api but when trying to use this with the feature gate at a different path, it's not presently working.

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[enxebre]

/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: enxebre, JoelSpeed

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