Enable running on .net core 3.0

[baronfel]

This tool will fail to run on .net core 3 runtimes, because the default roll-forward config only allows rolling-forward across minor versions. There are a few options here, in order from worst to best:

• let the user specify a roll-forward env variable: DOTNET_ROLL_FORWARD_ON_NO_CANDIDATE_FX=2 to allow major-version roll forward
• add a matching setting to this tool's runtimeconfig.json file, like I did in Paket: add roll-forward config to enable running on later major versions of the runtime Paket#3635
• do the work to multitarget the tool to netcoreapp3.0

I plan on issuing a PR soon for the second option, but I wanted to add this issue explicitly in case any FAKE users try to run on 3.0.

[matthid]

But this is a duplicate of #2325? The current workaround is already documented. It's just that I didn't knew option 2 existed.

[baronfel]

yep, thanks! closing as duplicate, I forgot you linked to that one in the other thread and I was on auto-pilot submitting this exact text to a few of my favorite dotnet tools :)