Fix netlink descriptor error retry

[bdice]

We added a retry for Unexpected error [0-9]+ on netlink descriptor [0-9]+ but grep's default syntax doesn't support that +. This adds the -E flag to make that work.

$ echo "Unexpected error 9 on netlink descriptor 4." | grep -q "Unexpected error [0-9]+ on netlink descriptor [0-9]+" && echo "matches!"
$ echo "Unexpected error 9 on netlink descriptor 4." | grep -qE "Unexpected error [0-9]+ on netlink descriptor [0-9]+" && echo "matches!"
matches!