lava/config.yaml: bump imgpath for v2025

The refactoring of the branching pipeline missed the update of the variable.

The LAVA jobs nfsroot-simple-boot are failing because the path to the image is wrong since it still uses an old version to define the path of the new image.

For instance, see LAVA logs at https://qa.apertis.org/report/v2025/20250205.0117/nfs