Fix copying add-ons to the system profile (#19511)
### Summary of the issue:
#19446 introduced the ability to select which add-ons should be copied
to the system profile (now described to users as "NVDA's system-wide
configuration"). Early in the prototype phase, I underscore-prefixed the
new `addonsToCopy` parameter to `setSystemConfigToCurrentConfig`. I
removed the underscore late in the dev/testing process, so late that I
didn't do enough testing to ensure that every instance was renamed.
### Description of user facing changes:
Copying configuration to the system profile works again.
### Description of developer facing changes:
None.
### Description of development approach:
Corrected the name of the parameter in
`gui.settingsDialogs.GeneralSettingsPanel.onCopySettings`. Searched
`source/` for any other instances of `_addonsToCopy` and found none.
### Testing strategy:
Created a dist. Installed from same. Copied settings to system settings,
with and without add-ons installed and selected.
### Known issues with pull request:
None
