ci: Pin to macos-26 runner for matching SDK

macos-latest still resolves to macos-15 (Sequoia). Developer is on
macOS 26 (Tahoe), so binaries built on macos-15 still trigger SwiftUI
behavior-compat shims affecting MenuBarExtra panel sizing on Tahoe
hosts. Pin to macos-26 directly so the build SDK matches the host OS
the developer (and likely most early Tahoe adopters) is running.