No-op tuist generate preserves project.pbxproj on disk

tuist generate no longer rewrites project.pbxproj when the freshly serialized bytes are identical to what is already on disk, preserving the file’s modification time. This stops unnecessary inode churn from showing up as spurious git diffs and file-watcher wakeups, and brings the pbxproj writer in line with the rest of the generated files writers. Real structural changes still rewrite the file exactly as before.