Set missing environment variable

[aveXcaesar]

Please check if the PR fulfills these requirements

• [x] I made sure that my code builds
• [x] I merged the master into this branch before pushing
• [x] The commit message follows our guidelines
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce?** (check one with "x")

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [x] Build related changes
• [ ] CI related changes
• [ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

closes #167

What is the new behavior?

• cake build fail when a environment variable is not set
• set missing environment variable in appveyor.yml

Does this PR introduce a breaking change? (check one with "x")

• [ ] Yes
• [x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications

Other information

[codecov[bot]]

Codecov Report

Merging #168 (aaf7106) into master (175b8df) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #168   +/-   ##
=======================================
  Coverage   54.32%   54.32%           
=======================================
  Files          38       38           
  Lines         959      959           
  Branches       87       87           
=======================================
  Hits          521      521           
  Misses        412      412           
  Partials       26       26           

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