add inst_vegetation_area_frac to export field for GOCART

[bbakernoaa]

Description

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Provide a detailed description of what this PR does.
What bug does it fix, or what feature does it add?
Is a change of answers expected from this PR?

Issue(s) addressed

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• fixes #<issue_number>
• fixes noaa-emc/fv3atm/issues/<issue_number>

Testing

How were these changes tested?
What compilers / HPCs was it tested with?
Are the changes covered by regression tests? (If not, why? Do new tests need to be added?)
Have the ufs-weather-model regression test been run? On what platform?

• Will the code updates change regression test baseline? If yes, why? Please show the baseline directory below.
• Please commit the regression test log files in your ufs-weather-model branch

Dependencies

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Those branches should have matching names (ideally)

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If so add the "waiting for other repos" label and list the upstream PRs

• waiting on noaa-emc/nems/pull/<pr_number>
• waiting on noaa-emc/fv3atm/pull/<pr_number>