jobsite/builder: enable --json

Use `--json`; this causes `osbuild` to buffer all output and then print
a huge blob of JSON at the end of the command.

cmd/osbuild-jobsite-builder/main.go

@@ -242,6 +242,7 @@ func (builder *Builder) HandleBuild(w http.ResponseWriter, r *http.Request) erro
 		"--store", path.Join(argBuildPath, "store"),
 		"--cache-max-size", "0",
 		"--output-directory", path.Join(argBuildPath, "export"),
+		"--json",
 	}
 
 	for _, pipeline := range buildRequest.Pipelines {