Added Gallery_Api Environment variable to AppVeyor - Fixes #385

[PlagueHO]

Pull Request (PR) description

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

This Pull Request (PR) fixes the following issues

• Fixes Example Publishing to PS Gallery Fails #385

Task list

• 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 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 - one more for you! Thanks Johann!

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This change is 

[codecov-io]

Codecov Report

Merging #386 into dev will not change coverage.
The diff coverage is n/a.

@@         Coverage Diff         @@
##            dev   #386   +/-   ##
===================================
  Coverage    95%    95%           
===================================
  Files        27     27           
  Lines      2144   2144           
===================================
  Hits       2053   2053           
  Misses       91     91

[johlju]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[39Delta]

LGTM