Add `exec-env` template with `TOOL_ROOT` env

[uvlad7]

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

Many plugins don't set TOOL_ROOT env variable(s), see hensou/asdf-dotnet#26, asdf-community/asdf-rust#32, rkyleg/asdf-julia#36; some used to set it the wrong way

Describe the solution you'd like

I'd be nice to add an exec-env template

Describe alternatives you've considered

Adding information about it into the README, but it seems less convenient and can be easily missed

Additional context