ci: rename PS_GALLERY_KEY secret reference to PSGALLERY_API_KEY (#51)

The env var fed to build.ps1 is already named PSGALLERY_API_KEY; this
brings the GitHub Actions secret name into the same convention. The new
PSGALLERY_API_KEY secret has been pre-populated with the same value as
PS_GALLERY_KEY so the next publish will succeed without intervention.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: tablackburn

.github/workflows/PublishModuleToPowerShellGallery.yaml

@@ -77,5 +77,5 @@ jobs:
         if: steps.check_psgallery.outputs.exists == 'false'
         shell: pwsh
         env:
-          PSGALLERY_API_KEY: ${{ secrets.PS_GALLERY_KEY }}
+          PSGALLERY_API_KEY: ${{ secrets.PSGALLERY_API_KEY }}
         run: ./build.ps1 -Task Publish -Bootstrap