Set LC_CTYPE to UTF-8 for dtests

Patch for CASSANDRA-14298
diff --git a/docker/jenkins/dtest.sh b/docker/jenkins/dtest.sh
index 0d8cdb9..20915b6 100644
--- a/docker/jenkins/dtest.sh
+++ b/docker/jenkins/dtest.sh
@@ -1,6 +1,7 @@
 #!/bin/sh
 export WORKSPACE=/home/cassandra/cassandra
 export LANG=en_US.UTF-8
+export LC_CTYPE=en_US.UTF-8
 export PYTHONIOENCODING=utf-8
 export PYTHONUNBUFFERED=true
 echo "dtest.sh: running: git clone --depth=1 --branch=$BRANCH https://github.com/$REPO/cassandra.git"