fix(ci): remove invalid on_first_boot field from answer.toml

The on_first_boot option is a CLI flag for prepare-iso, not a valid
field in the answer file's [global] section. The first-boot script
is already passed via --on-first-boot in prepare-auto-iso.sh.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/infrastructure/answer.toml.tftpl

@@ -7,7 +7,6 @@ timezone = "UTC"
 root_password = "${root_password}"
 reboot_on_error = true
 root_ssh_keys = []
-on_first_boot = "first-boot.sh"
 
 [network]
 source = "from-dhcp"