ci: update image_type defaults, validate required secrets set perms

Address review comments from copilot and Brett:
- update setup-lxd pinning
- validate presence of non-empty required SSH_*_KEY secrets
- simplifiy -z secret test conditionals to avoid multi-line issues
- use install -m 600 before creating SSH_PRIVATE_KEY file
- test and exit 1 on non-encoded PYCLOUDLIB_TOML repo secret
- set image_type: generic as defaults in 100-dispatch-common and ec2
  job.