Xcode: View > Navigators > Reports
The Report Navigator is the “chat bubble” in the Xcode sidebar.

Select your most recent Build from the sidebar on the left.
Then click All Messages in the tab bar to see the output of your Run Script Build Phase.

Xcode Version changes:
-
Xcode 12:
View > Navigators > Reports -
Xcode 6:
View > Navigators > Show Report Navigator -
Xcode 5:
View > Navigators > Show Log Navigator -
Xcode 4:
View > Navigators > View Log Navigator