tuist generate skips rewriting project.pbxproj when output is unchanged

Running tuist generate no longer rewrites project.pbxproj when the newly serialized project is byte-identical to the file already on disk. Because Tuist generation is deterministic for an unchanged input, a no-op regeneration now leaves the pbxproj untouched, preserving its modification time and inode. This avoids spurious git status churn and unnecessary file-watcher wakeups between builds. When you actually make a structural change, the file is rewritten just like before, so behavior on real regenerations is unchanged. Scope is limited to project.pbxproj; the workspace and scheme writers still rewrite unconditionally.