travis: disable the cert callback on OSX

The sshd on OSX has stopped responding to the setting which this test
assumes is there, so let's disable it until we can figure out what's
happening.

script/cibuild.sh

@@ -43,6 +43,9 @@ export GITTEST_REMOTE_SSH_PUBKEY="$HOME/.ssh/id_rsa.pub"
 export GITTEST_REMOTE_SSH_PASSPHRASE=""
 
 if [ -e ./libgit2_clar ]; then
-    ./libgit2_clar -sonline::push -sonline::clone::cred_callback -sonline::clone::ssh_cert &&
+    ./libgit2_clar -sonline::push -sonline::clone::ssh_cert &&
     ./libgit2_clar -sonline::clone::ssh_with_paths
+    if [ "$TRAVIS_OS_NAME" = "linux" ]; then
+        ./libgit2_clar -sonline::clone::cred_callback
+    fi
 fi