Update `DEVGUIDE.md` how to use plain `dotnet build` with non-released SDKs

[vzarytovskii]

Currently, Arcade will update sdk in global.json on non-released builds (at the moment of posting it's 10p6, where only 10p5 was released).

Now if one tries to build FSharp.Compiler.Service.sln, the command will yield the following:

> dotnet build ./FSharp.Compiler.Service.sln
The command could not be loaded, possibly because:
  * You intended to execute a .NET application:
      The application 'build' does not exist or is not a managed .dll or .exe.
  * You intended to execute a .NET SDK command:
      The .NET SDK could not be found, please run ./eng/common/dotnet.sh.

The second advise is not very visible, can it be added to DEVGUIDE.md, advising people to run ./eng/common/dotnet.sh before the plain build?
Additionally, a dev container can be updated to run the dotnet.sh script before the building the repo, additionally, setting DOTNET_ROOT to the locally downloaded one, as well as adding it to PATH, so all tooling "just works" in dev container.

Also, when running this command currently, I'm getting the following: