SpaProxy should not inject middleware with ASPNETCORE_HOSTINGSTARTUPASSEMBLIES

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Describe the bug

I wanted to post in this bug: https://github.com/dotnet/aspnetcore/issues/45130

But it's been locked to me even though I created it. I was asked there by @javiercn as to what problems I run into with the new ASP.NET SPA approach that puts express.js in front of the ASP.NET server, so for the record I'd like to note two things:

1) I like to be able to modify the index.html page in the pipeline so that I can inject JavaScript into the page to provide configuration information from the ASP.NET app
2) I like to be able to selectively inject JS into the index.html page so that I can eg. only inject Google Analytics on the production server.

It seems to me that unless ASP.NET has ultimate control over serving the index.html page, I'm unable to do that with the new mechanism.

### Expected Behavior

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### Steps To Reproduce

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### Exceptions (if any)

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### .NET Version

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### Anything else?

_No response_