Stat Binary/Extend after downloading from Src or simply returning local Src

[korenyoni]

On local Extrend retrievals, os.Stat is performed on the Src which is returned (https://github.com/yonkornilov/snowcapper/blob/master/runner/runner.go#L132-L138). This is not done on remote retrievals.

For Binary retrievals, the os.Stat operation is done in neither remote or local retrievals, and should be done for both.

Renaming downloadPath in getBinary and getExtract to something in favour of local retrievals should be done too.