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

Commit bc9094f4 broke the first stage in many ways, and commit 3b9a1487 forgot to fix the path to the DTB files needed for the ARM64 Renesas R-Car boards, resulting in the error below:

Job error: Invalid job data: ["Resource unavailable at 'https://images.apertis.org/release/v2019/v2019.1/arm64/dtbs/renesas/r8a7796-m3ulcb.dtb' (404)"]

Broken job:

• https://lava.collabora.co.uk/scheduler/job/2173082

Tested as:

$ lqa submit -g ./apertis-tests/templates/profiles.yaml \
    --profile apertis-minimal-arm64-uboot \
    -t release:v2021dev1 \
    -t image_date:20200113.0 \
    -t image_name:apertis_v2021dev1-minimal-arm64-uboot_20200113.0 \
    ./apertis-tests/templates/group-minimal-tpl.yaml 
Submitted job group-minimal-tpl.yaml with id 2173825

Fixed job:

• https://lava.collabora.co.uk/scheduler/job/2173825