Added Gallery_Api Environment variable to AppVeyor - Fixes #202

[PlagueHO]

Pull Request (PR) description

This PR fixes the example publish to PS Gallery by adding the missing environment variable to

This Pull Request (PR) fixes the following issues

• Fixes #202

Task list

• [x] Added an entry under the Unreleased section of the change log in the CHANGELOG.md. Entry should say what was changed, and how that affects users (if applicable).
• [ ] Resource documentation added/updated in README.md in the resource folder.
• [ ] Resource parameter descriptions added/updated in schema.mof and comment-based help.
• [ ] Comment-based help added/updated.
• [ ] Localization strings added/updated in all localization files as appropriate.
• [ ] Examples appropriately added/updated.
• [ ] Unit tests added/updated. See DSC Resource Testing Guidelines.
• [ ] Integration tests added/updated (where possible). See DSC Resource Testing Guidelines.
• [ ] New/changed code adheres to DSC Resource Style Guidelines and Best Practices.

@johlju - would you mind reviewing? Should be a quick one.

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This change is 

[codecov-io]

Codecov Report

Merging #203 into dev will not change coverage. The diff coverage is n/a.

@@        Coverage Diff         @@
##           dev   #203   +/-   ##
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  Coverage   94%    94%           
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  Files        8      8           
  Lines      889    889           
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  Hits       837    837           
  Misses      52     52