fix: preserve staged update dir when pruning orphaned update dirs on macOS
The previous squirrel.mac patch cleaned up all staged update directories
before starting a new download. This kept disk usage bounded but broke
quitAndInstall() if called while a subsequent checkForUpdates() was in
flight — the already-staged bundle would be deleted out from under it.
This reworks the patch to read ShipItState.plist and preserve the
directory it references, deleting only truly orphaned update.XXXXXXX
directories. Disk footprint stays bounded (at most 2 dirs: staged +
in-progress) and quitAndInstall() remains safe mid-check.
Also adds test coverage for the quitAndInstall/checkForUpdates race and
a triple-stack scenario where 3 updates arrive without a restart.
Refs https://github.com/electron/electron/issues/50200
6be775ad83
