document two more failures

Author: sylvestre

util/why-error.txt

@@ -1,8 +1,8 @@
 This file documents why some tests are failing:
 
-gnu/tests/chgrp/from.sh
+gnu/tests/chgrp/from.sh - https://github.com/uutils/coreutils/issues/7039
 gnu/tests/chmod/symlinks.sh - https://github.com/uutils/coreutils/pull/7025
-gnu/tests/chroot/chroot-credentials.sh
+gnu/tests/chroot/chroot-credentials.sh - https://github.com/uutils/coreutils/issues/7040
 gnu/tests/cp/cp-i.sh
 gnu/tests/cp/preserve-gid.sh
 gnu/tests/csplit/csplit-suppress-matched.pl