WIP: Move unsupported platform failure from os.name to separate method.

[deepy]

Previously we've accidentally depended on the isWindows check to see if we're on a supported platform or not, this had the unfortunate side-effect that even with download = false we triggered the error()

I've moved this to PlatformHelper.failIfUnsupportedPlatform()

[deepy]

Hmm, this wasn't enough, there's still issues

Could not determine the dependencies of task ':nodeSetup'.
> Failed to query the value of task ':nodeSetup' property 'nodeArchiveFile'.
   > Could not resolve all files for configuration ':detachedConfiguration1'.
      > Could not find node-14.15.4-unsupported-x64.tar.gz (org.nodejs:node:14.15.4).
        Searched in the following locations:
            https://nodejs.org/dist/v14.15.4/node-v14.15.4-unsupported-x64.tar.gz

[deepy]

this work isn't abandoned, but currently it looks like it might be easier to solve in a different way