While Xcode 9 is beta, I have the typical arrangement,

however, when I carthage update indeed it tries to use the Xcode8 tools, so I get:

is there a way to tell Carthage to build with the "other" Xcode?
If you have multiple versions of the Apple developer tools installed (an Xcode beta, for example), use xcode-select to change which version Carthage uses.
https://github.com/Carthage/Carthage#resolve-build-failures
Excellent, thanks. For anyone googling here, it turns out you can also simply change this in the Xcode preferences.
Just open the newer Xcode and do this:

Most helpful comment
Excellent, thanks. For anyone googling here, it turns out you can also simply change this in the Xcode preferences.
Just open the newer Xcode and do this: