PowerShell 5.1 strips an empty-string argument ('') when invoking a native
exe, so `reg add ... /d '' /f` reached reg.exe as `reg add ... /d /f`: reg
consumed /f as the data value, lost the force flag, and prompted
"overwrite? (Yes/No)" on stdin. FirstLogon runs headless with no stdin, so
post-install hung forever right after the "Explorer UX tweaks applied" log
line. Route both empty-value reg adds (HKCU + Default hive) through
`cmd /c`, which preserves the empty "" so the value is set and /f stays.
Regression from 8095f00 (wallpaper removal feature).