opam requires rsync for local VCS pins [2.1 regression]

[dra27]

Since opam 2.1, pinning a local VCS repository requires rsync:

"rsync": command not found.

This is the same error message as a local pin would give, when rsync isn't available, but it's rather more confusing given that opam init claimed that the git backend is available (and it does indeed for non-local pins).

I expect the best fix is to improve the error message (modulo some of the other pending issues with OpamVCS using rsync).