/usr/bin/cert-sync is in ca-certificates-mono 4.2.1.102+dfsg2-7ubuntu4.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 | #!/bin/sh
exec /usr/bin/mono $MONO_OPTIONS /usr/lib/mono/4.5/cert-sync.exe "$@"
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