Fix self-contained packaging.

[pchote]

The .NET docs noted that the --self-contained parameter would

Default is true if a runtime identifier is specified and the project is an executable project (not a library project).

I initially interpreted this as meaning it would default to true for all the projects in the sln, because it would be crazy if some projects were treated as self contained and others weren't when all are being published together. It turns out that .NET is crazy, and that the csproj changes in #18995 introduced new breaking errors related to this (thanks to @IceReaper for discovering the issue).

Setting --self-contained explicitly fixes this, and also appears to solve the trimming problems from the original PR, so i'm dropping those workarounds too.

Test release: https://github.com/pchote/OpenRA/releases/tag/devtest-20210309

[IceReaper]

I can confirm that the packaging works. However...
Windows: When running the installer, it basicaly spamt error at the end of the installation process for 1-2 minutes:

AppImage running dedicated server: It directly crashes on start with

File name: 'System.Runtime, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
System.IO.FileNotFoundException: Could not load file or assembly 'System.Runtime, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. The system cannot find the file specified.

[pchote]

That is caused by OpenRA/OpenRAModSDK#171 (comment), so nothing to worry about here.

[Mailaender]

Works fine on Linux. I don't know what to changelog here though.

[pchote]

https://github.com/OpenRA/OpenRA/wiki/Changelog-(bleed)/_compare/9f6eb5e5e1ee58c08ea6eaa7c684392ec6d5a112