Checks for posix, per windows can not install posix

[mikehancoski]

Fix for this issue
#7

[greg-1-anderson]

Is it best to assume interactivity or non-interactivity here? It it going to cause problems if the wrong decision is made here?

It would be better if --no-interaction would be checked / respected, so the user would retain some control over behavior when automatic detection is not available.

[ronan]

Is it best to assume interactivity or non-interactivity here?

I'm not sure it matters since the command doesn't have an interactive mode. Currently it can only read a list of sites from STDIN.

This pr will prevent the error but I'm assuming it won't actually make the command work on Windows. I have no idea what you would need to do to be able to do this on Windows.

I'm happy to merge this to prevent a fatal error, but I would rather find a way to either make it work, or return an error stating that the command doesn't work on Windows.

[mikehancoski]

I do not know of a way to check for non-interactivity under windows.

I tested this under windows, it appears functions but throws a warning

Notice: Undefined property: stdClass::$update_log in C:\Program Files (x86)\Git
vendor\pantheon-systems\terminus\src\Commands\Upstream\Updates\UpdatesCommand.ph
p on line 42

Then it says that updates were applied, I assumed that since my sites were updated this worked. However it appears that one of my co workers ran the update command (under OSX) and my update did not actually complete.(under windows)

I may revisit this and add an else that replicates the original check with posix.