lava/config.yaml: bump imgpath for v2026dev1

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.

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>