Compilation fails with BSD `sed`

[yforster]

As just found out in a debugging session with @dominik-kirst, the ${SED} line in update_plugin.sh removes the letter r from cRelationClasses.mli instead of removing the carriage \r if when using BSD sed This is different from the usual problem that BSD sed requires -i.bak instead of just -i and I have no idea what's really going on, but wanted to document the problem. The Mac OS version used is 10.15.7 with Coq 8.13 on OCaml 4.07.1 using the beta2+8.13 release.