lava: Fix typo and prevent similar issues

The baseurl variable in the template was misspelt as base_url when the template was instantiated, resulting in LAVA jobs failing due to the invalid URLs generated:

Job error: Unsupported url protocol scheme:

Use set -u to ensure the shell loudly complains if we end up again using a variable that has not been explicitly set.

Broken job: https://lava.collabora.co.uk/scheduler/job/2872302 Fixed job: https://lava.collabora.co.uk/scheduler/job/2873295