build_flag: improve default flag value directory

The default directory for flag values is the last of:
 - Where the flag is declared,
 - Where the release config is first declared,
 - Where the flag value is currently being set for this release config.

Bug: 328495189
Test: manual
Change-Id: Ida1f906b75704f658e7fa1de703e0c789dce06a6

cmd/release_config/build_flag/main.go

@@ -266,7 +266,7 @@ func SetCommand(configs *rc_lib.ReleaseConfigs, commonFlags Flags, cmd string, a
 		return fmt.Errorf("Unknown build flag %s", name)
 	}
 	if valueDir == "" {
-		mapDir, err := GetMapDir(*flagArtifact.Traces[len(flagArtifact.Traces)-1].Source)
+		mapDir, err := configs.GetFlagValueDirectory(release, flagArtifact)
 		if err != nil {
 			return err
 		}