dotnet run OR dotnet watch with development environment from command line?

ASPNETCORE_ENVIRONMENT is an environment variable (and AFAIK) not a switch to the dotnet cli.

So what you would do is set it prior to using the tool:

rem Windows
C:\> set ASPNETCORE_ENVIRONMENT=Development
C:\> dotnet ...

rem Unix
$ export ASPNETCORE_ENVIRONMENT=Development
$ dotnet ...