Fix runtime sync script (#46672)

.github/workflows/runtime-sync.yml

@@ -60,7 +60,7 @@ jobs:
         # Test this script using an issue in the local forked repo
         $issue = 'https://api.github.com/repos/dotnet/aspnetcore/issues/18943'
         $sendpr = .\aspnetcore\.github\workflows\ReportDiff.ps1
-        echo "sendpr=$sendpr" >> $GITHUB_OUTPUT
+        echo "sendpr=$sendpr" >> $env:GITHUB_OUTPUT
     - name: Send PR
       if: steps.check.outputs.sendpr == 'true'
       # https://github.com/marketplace/actions/create-pull-request