Our Sparkle build is not hermetic

[mherrmann]

Brave's build uses a hermetic copy of Xcode in certain environments (at the moment: when Goma is enabled). Chromium has an arg use_system_xcode that gets set to false in those cases. The problem is that our copy of Sparkle does not respect these settings. It simply invokes xcodebuild. This is not (currently) included in the hermetic copy of Xcode, thus gets picked up from the PATH, and thus from the system-installed Xcode.