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CPack/NSIS: Fix silent NSIS installs

The installer should not put up a message box to confirm uninstall of a
previous version for silent installs. Furthermore, when the uninstaller
is invoked, do not show its confirmation dialog because either (a) the
installer already asked the user or (b) the installer is running in
silent mode.

Closes: #16328
1 job for silent-nsis-installs
Status Job ID Name Coverage
  External
passed kwrobot-commit-check