plasma-nm: fix path to mobile broadband provider file (#44302)

When creating a new mobile broadband connection
with the plasma network manager connection editor,
it tries to find a file containing provider
information somewhere in /usr/share/... .
The build recipe contains a patch to fix the lookup path
such that it finds the file in the corresponding package,
probably added due to
https://github.com/NixOS/nixpkgs/issues/9389 .
The actual lookup path is injected into
the patch file with substituteAll.

With commit a31d98f3 ,
the variable name used in subsituteAll changed from
mobile_broadband_provider_info to mobile-broadband-provider-info
(underscores in package names turned into dashes).
Apparently, substituteAll can't handle dashes in variable names.
Consequently, the variable name was no longer resolved.
plasma-nm failed to create new mobile broadband connections;
the connection creator silently exited and logged the error
> plasma-nm: Error opening providers file "@mobile-broadband-provider-info@/share/mobile-broadband-provider-info/serviceproviders.xml"

This commit keeps the dashes in package names, but it
restores the underscores in the variable used by substituteAll,
thereby ensuring the variable gets resolved properly.