Web CI: make multi-browser test job optional (microsoft#21669)

### Description

This job is a little bit unstable. Make it optional to avoid blocking
other PRs before we revise it.

Author: fs-eire

tools/ci_build/github/azure-pipelines/templates/win-web-multi-browsers.yml

@@ -8,6 +8,7 @@ jobs:
   pool:
     vmImage: windows-2019
   timeoutInMinutes: 60
+  continueOnError: true
   workspace:
     clean: all
   steps: