fix(pkl): bind ArtifactConfig.Repositories as []*Repository

PKL's Listing<ClassType> decodes into Go via reflection as a slice of
pointers, not values. Declaring the field as []Repository panicked with
"reflect.Set: value of type *model.Repository is not assignable to type
model.Repository" the first time PKL produced a non-empty repositories
listing. Match the pattern used elsewhere (e.g., FilterCondition
conditions) and use a pointer slice. The translation layer at
pkl.go:720 reads through the pointer transparently.

Author: JeroenSoeters

internal/schema/pkl/model/config.go

@@ -184,7 +184,7 @@ type ArtifactConfig struct {
 	URL          url.URL      `pkl:"url"`
 	Username     string       `pkl:"username"`
 	Password     string       `pkl:"password"`
-	Repositories []Repository `pkl:"repositories"`
+	Repositories []*Repository `pkl:"repositories"`
 }
 
 type CliConfig struct {