templates: Fix the path to the NFS first stage after the switch to v2019.1

Commit bc9094f4 broke the first stage, see for instance https://lava.collabora.co.uk/scheduler/job/2166454

Job error: Invalid job data: [ "Resource unavailable at 'https://images.apertis.org/release/v2019/v2019.1/amd64/initrd.img' (404)", "Resource unavailable at 'https://images.apertis.org/release/v2019/v2019.1/amd64/apertis-nfs-amd64.tar.gz' (404)"]

A nfs/ component in the path was missing.

Test job:

• lqa submit -g ./apertis-tests/templates/profiles.yaml --profile apertis-minimal-amd64-uefi -t release:v2021dev1 -t image_date:20200110.0 -t image_name:apertis_v2021dev1-minimal-amd64-uefi_20200110.0 ./apertis-tests/templates/group-minimal-tpl.yaml
• https://lava.collabora.co.uk/scheduler/job/2168939