add comment to explain why NO_MOBILE=true

Author: jpnurmi

.github/actions/environment/action.yml

@@ -11,6 +11,7 @@ runs:
         echo "DOTNET_CLI_TELEMETRY_OPTOUT=1" >> $GITHUB_ENV
         echo "DOTNET_NOLOGO=1" >> $GITHUB_ENV
 
+    # Android SDK is not available
     - name: Set Environment Variables for Alpine Linux
       if: ${{ contains(matrix.container, 'alpine') }}
       shell: bash