Fix MessagePackWindow can not run mpc command

[neuecc]

After this commit 1586a4d
ToolCommandName was changed from dotnet-mpc to mpc.
MessagePackWindow invoke dotnet mpc that expect to install by global tools so does not work.

@AArnott
By the way, is don't use dotnet- for ToolCommandName the standard style?

[AArnott]

By the way, is don't use dotnet- for ToolCommandName the standard style?

For global tools, that's certainly true. I think for project-level installed tools that you'd have to use dotnet in front of the command name.
mpc may be either global or project level. It depends on how it is installed.