xcbuild: only override version with 1 arg

sometimes we want the "SDK" version from xcbuild so we do something
like:

$ xcbuild -version -sdk MacOSX10.10
SDKSettings.plist - MacOSX10.10 (MacOSX10.10)
SDKVersion: 10.10
Path: /nix/store/6k7crm1n4drf09ga0dwvbmb59x4zl2i2-SDKs/MacOSX10.10.sdk
PlatformPath: /nix/store/vhfwb1znfy65s2xs27j8xribk6mp6lbw-Platforms/MacOSX.platform
ProductName: Mac OS X
ProductVersion: 10.10

This was previously overriden by the current xcode version so you
would get:

Xcode 9.4.1
Build version 17E189

This should fix the other usage of -version in nodejs 6.x.