Define Content-Type on apiHeaders for HTTP requests to GitHub

[kNoAPP]

As of a few days ago, this action started throwing the exception:

Invoke-WebRequest: /home/runner/work/_actions/zyborg/gh-action-buildnum/v1/action.ps1:158
Line |
 158 |  … eGistResp = Invoke-WebRequest -Headers $apiHeaders -Uri $updateGistUr …
     |                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     | The given key 'Content-Type' was not present in the dictionary.

This PR fixes that error.