clan-cli: Fix disko template to not fail because of missing bootloader.

pkgs/clan-cli/clan_cli/api/disk.py

@@ -138,6 +138,7 @@ def set_machine_disk_schema(
     # Use get disk schemas to get the placeholders and their options
     placeholders: dict[str, str],
     force: bool = False,
+    allow_uknown_placeholders: bool = False,
 ) -> None:
     """
     Set the disk placeholders of the template
@@ -182,7 +183,11 @@ def set_machine_disk_schema(
             )
 
         # Invalid value. Check if the value is one of the provided options
-        if ph.options and placeholder_value not in ph.options:
+        if (
+            ph.options
+            and placeholder_value not in ph.options
+            and not allow_uknown_placeholders
+        ):
             msg = (
                 f"Invalid value {placeholder_value} for placeholder {placeholder_name}"
             )