Make it easy to detect if a .NET app is running as a .NET SDK Tool

### Is your feature request related to a problem? Please describe.

It can be helpful as a app author to know if my app is running as a .NET Tool or not, but the SDK doesn't currently give me any signals that this is the case. I can technically see if my app's actual launching binary is `dotnet`, but that only works for local tools or tools run via `dnx` - global tools use apphost shims that don't surface `dotnet` at all.

### Describe the solution you'd like
The CLI should ensure that when tools are run, a marker environment variable is available in the spawned process . This environment variable should be called `DOTNET_TOOL_EXECUTION_MODE` and it should be set to one of the following values:

* `oneshot` when the tool is run via `dnx`, `dotnet tool execute`, or `dotnet dnx` (which are actually all the same mechanism, just with different names for _reasons_)
* `local` when the tool is run via `dotnet ` or `dotnet tool run ` as a local tool
* `global` when the tool is run via `` with it's globally-installed apphost shim, or `dotnet ` when the global shim is named `dotnet-`

This will be easier for `oneshot` and `local` tools, because the CLI owns the spawning of those processes. For global tools, we'll likely have to find a way to embed the env var into the script-or-apphost that's launching the app, and I don't know if that capability exists out of the box with the apphosts that we currently use.