CPack/NSIS: Fix CPACK_NSIS_UNINSTALL_NAME when not Uninstall.exe

If a previous installation was detected by the NSIS script the
uninstaller was not correctly run unless it was still named
`Uninstall.exe`.  This meant that using `CPACK_NSIS_UNINSTALL_NAME`
was simply broken.

Fixes: #22103