Cvpcb unaware of project context when started from kicad toplevel

When cvpcb is started from the project overview by opening a .net file, the project context is not properly communicated.

To reproduce:

1. Create a new project
2. Create a simple schematic
3. Create netlist from schematic
4. Exit eeschema, double-click .net file in project overview
5. Open the Preferences -> Edit Library Table dialog in cvpcb
6. Switch to the "Project Specific Libraries" tab, and create a test entry

Observed behaviour: this entry is saved to the file ~/prj-fp-lib-table
Expected behaviour: the entry should be saved to "fp-lib-table" inside the project directory

This suggests that the project context is not properly communicated to the cvpcb instance.

When cvpcb is invoked from within eeschema, the library path information is saved correctly.

The pcbnew program also uses the correct file when invoked from the project overview.