Regression: Not detecting HTTP_PLATFORM_PORT in IIS/Express

[Tratcher]

Regressed by #453
The configuration is now filtering out environment variables that do not begin with "ASPNET_". .AddEnvironmentVariables(prefix: EnvironmentVariablesPrefix).

Either re-work the config to pick up other environment variables, or read HTTP_PLATFORM_PORT directly from the Environment (not recommended as this makes testing more difficult).
@davidfowl

[davidfowl]

Did we add a test?

[pakrym]

@davidfowl nope, it's environment variable and we cannot test them