Reintroducing Get-BinRoot

This is the solution for (broken) packages which all have their own '`$binRoot` calculation' code.

Also adds transition to `CamelCaseEnvVar` to make things look cleaner. (Can be removed if this is not actively enforced.)

I know this will become part of `Install-ChocolateyPortablePackage`, but sometimes you just want to know the `binRoot`. (And never look at something that _"should be handling things at some point in the future,"_ only look at _"is this better than how this is done now"_ :P )