Don't change unspecified gid to 0 during chown

Specifying 0 for gid as a default during chown causes the
gid to be changed to 0 when left unspecified. Changing the
default value to -1 causes the gid to be left unmodified
when unspecified, which is a less surprising behavior.

Signed-off-by: Jonathon Anderson <janderson@ciq.co>

internal/app/wwctl/overlay/chown/main.go

@@ -34,7 +34,7 @@ func CobraRunE(cmd *cobra.Command, args []string) error {
 			os.Exit(1)
 		}
 	} else {
-		gid = 0
+		gid = -1
 	}
 
 	overlaySourceDir = overlay.OverlaySourceDir(overlayName)