Use 'gpr' to push to GitHub Package Registry.

This is to try to avoid the following warnings from the workflow:

WARNING: No API Key was provided and no API Key could be found for 'https://nuget.pkg.github.com/mysql-net'. To save an API Key for a source use the 'setApiKey' command.
Pushing MySqlConnector.1.3.12-beta.0.5.nupkg to 'https://nuget.pkg.github.com/mysql-net'...
  PUT https://nuget.pkg.github.com/mysql-net/
WARNING: Please use the --api-key option when publishing to GitHub Packages

This solution comes from https://github.community/t/github-package-registry-not-compatible-with-dotnet-nuget-client/14392/19.

Author: bgrainger

.github/workflows/publish-package.yml

@@ -36,15 +36,12 @@ jobs:
     - name: Pack
       run: dotnet pack --configuration Release --no-build
 
-    - name: Install NuGet
-      run: choco install nuget.commandline
-
-    - name: Add NuGet source
-      run: nuget sources Add -Name "GPR" -Source "https://nuget.pkg.github.com/mysql-net/index.json" -UserName bgrainger -Password ${{ secrets.GITHUB_TOKEN }}
+    - name: Install gpr tool
+      run: dotnet tool install -g gpr --no-cache -v q
 
     - name: Publish packages
       run: |
         $files = Get-ChildItem -Path src -Filter *.nupkg -Recurse
         foreach ($file in $files) {
-          nuget push $file -Source GPR
+          gpr push --api-key $GITHUB_ACCESS_TOKEN $file
         }