Make UseEnvironment/ContentRoot override config

[Tratcher]

dotnet/aspnetcore#18499

UseEnvironment and UseContentRoot have a composition problem. Because they were implemented using config they're order sensitive and can be overridden by config. That has caused confusion especially for UseEnvironment.

Proposal: Have UseEnvironment and UseContentRoot store their data in IHostBuilder.Properties rather than config and treat this as higher priority than config. This works well for UseEnvironment, but there are a few places where UseContentRoot is used to provide a default value that is intended to compose with config. Those can be easily expressed using ConfigureHostConfig like UseContentRoot used to.

There's at least one more UseContentRoot call in UseWindowsServices we might want to update.
https://github.com/dotnet/extensions/blob/f4066026ca06984b07e90e61a6390ac38152ba93/src/Hosting/WindowsServices/src/WindowsServiceLifetimeHostBuilderExtensions.cs#L50

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[Tratcher]

Interesting question if this breaks these scenarios:
https://github.com/dotnet/aspnetcore/blob/bbb851e3ebf40f79531bc13dd5c1b56b332237fc/src/Servers/IIS/IIS/src/WebHostBuilderIISExtensions.cs#L36
https://github.com/dotnet/aspnetcore/blob/d0ab959c89a0adf6b86d35039b6a91349a5bedd1/src/Hosting/TestHost/src/WebHostBuilderExtensions.cs#L126
They're each using the IWebHostBuilder variant of the API, but they wouldn't be able to override the content root if someone and called IHostBuilder.UseContentRoot before them.

All of the composition concerns are with UseContentRoot so we could leave that one alone, but it seems strange for the two to be different.

[danmoseley]

Hello, changes into this branch must meet bug bar and require a template. Fwded email.

Is this a port from master?