Prevent false-positive in EnvironmentHelper.isMacOS on Windows

[allykzam]

If the current-directory is on a mapped drive, e.g. the M:\ drive, calling File.Exists with "/usr/bin/osascript" translates to "M:\usr\bin\osascript" -- this exists in some cases where Windows is running as a virtual machine on an OSX host.

Lots of code-comment to make this clear. Should still cover #1019 while also resolving #1202.

[inosik]

I think changing the check to

let isMacOS =
    (Environment.OSVersion.Platform = PlatformID.Unix) && // <-- Change OR to AND and OSX to Unix
      // osascript is the AppleScript interpreter on OS X
      File.Exists "/usr/bin/osascript"

should be enough.

The problem is that on Mono, Environment.OSVersion.Platform always returns Unix. This property never returned MacOSX, actually. See #1019 and #1044.

[inosik]

isMacOS = isUnix && File.Exists "/usr/bin/osascript"

would be even simpler.

[allykzam]

@inosik It would be simpler, but it would also behave incorrectly later if anyone ever felt like fixing mono (or .NET Core?) to correctly return MacOSX.

[inosik]

@amazingant you're right, didn't think about this.

Speaking of .NET Core, this API is (probably?) going to change anyways. I remember an issue about it at dotnet/corefx.