cube: incorrect use of reverse_binder in are_equivalent

Thanks to A. Martin for reporting this.

* spot/twacube_algos/convert.cc,
tests/core/twacube.cc: Here.
9 jobs for er/tmp in 95 minutes (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #93469
alpine-gcc

00:55:03

passed #93470
arch-clang

00:47:32

passed #93471
arch-gcc-glibcxxdebug

01:18:58

passed #93468
debian-gcc-snapshot

01:32:22

passed #93466
debian-stable-gcc

01:20:38

passed #93467
debian-unstable-gcc-coverage

00:59:11

89.6%
passed #93472
mingw-shared

00:10:27

passed #93473
mingw-static

00:09:15

passed #93474
armv7
raspbian

01:34:58