main: fix creating output dir for --with-buildlog

Trivial fix for the missing `mkdir()` call when `--with-buildlog`
is specified (and the matching test update).

cmd/image-builder/main_test.go

@@ -401,7 +401,7 @@ func TestBuildIntegrationArgs(t *testing.T) {
 		},
 	} {
 		t.Run(strings.Join(tc.args, ","), func(t *testing.T) {
-			outputDir := t.TempDir()
+			outputDir := filepath.Join(t.TempDir(), "output")
 
 			cmd := []string{
 				"build",