networkmanager-l2tp: hoist pname attribute

[danieldk]

Motivation for this change

Since #49398 was merged, name should have pname as a suffix, however
this derivation adds '-gnome' as a suffix when it is built with GNOME
support. This change makes the derivation more similar to some other
Network Manager plugins, where pname is not an attribute.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.

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[Mic92]

#49398 was reverted until we have a fix for all evaluation errors.

[danieldk]

@Mic92 indeed, I'll close this.