msolve -h call exits with non-0 code

This is a regression in 0.6.5 compared to 0.5.0.

With 0.5.0 one has
```
$ if $(msolve -h >/dev/null); then echo ok; else echo oops; fi
ok
```
but with 0.6.5, no
```
$ if $(msolve -h >/dev/null); then echo ok; else echo oops; fi
oops
```

Found while working on https://github.com/sagemath/sage/pull/37673 