source_completion.py re-execs itself into PM's selected interpreter before
touching any product, and the passthrough argv only carried --desktop. So
the startup heal's --finish-update was dropped, and the prepared invocation
fell through to the install tail: an update reported itself as an install.
✓ Install complete! [main @ c4056e3d6b]
Every flag that decides WHICH tail runs has to survive that boundary.
Verified with a stub that captures the bootstrap command instead of running
it: --finish-update present in the re-exec, --desktop still passed, and
without the flag the install path is unchanged. File compiles.
dbe4bfbc06