--interpolate always outputs instance ufo inside master-dir

The behaviour of `--interpolate` and the output location seems to have change with the last version bump and now erroneously always places UFO instances inside the master-dir.

This used to the be output:
```
fontmake -g font.glyphs -u --interpolate --master-dir=masters --instance-dir=instances

/masters/
/instances/
```

The same now (v. 2.3.0) results in:

```
/masters/
/masters/instances/
```

And without passing any dirs, too, the instance output is nested inside the master dir:
```
fontmake -g font.glyphs -u --interpolate
/master_ufo/ (default)
/master_ufo/instance_ufo/
/instance_ufo/ (gets created, but empty)
```

I'm using python 3.9.0 on Mac 11.1, and the installed dependencies are fonttools 4.18.2 and ufo2ft 2.19.2