Fallback to global.json if no version specified

[damccorm]

Fixes #14

[stephenmichaelf]

This is a non-trivial feature to add, @chrispat to approve from PM side.

[jviau]

What about introduce another variable dotnet-global-json and allow the user to point to the global.json file they want the task to use. dotnet-version and dotnet-global-json can be mutually exclusive.

[ZEisinger]

What about introduce another variable dotnet-global-json and allow the user to point to the global.json file they want the task to use. dotnet-version and dotnet-global-json can be mutually exclusive.

@jviau this was addressed in the discussion on the issue, and we will leave this out for now: #14 (comment)

Closing this PR in favor of using the install scripts for finding the version.