Ninja error building fonts. ninja: error: build.ninja:57: multiple rules generate

Hi team!

Trying to build fonts I get this error:

```
(venv) ➜ Montserrat git:(merge-forked-changes) gftools builder sources/config.yaml
ninja: error: build.ninja:57: multiple rules generate ../fonts/variable/Montserrat[wght].ttf

Removed file: /Users/juan/projects/Montserrat/sources/build.ninja
Removed directory: /Users/juan/projects/Montserrat/sources/master_ufo
Done building fonts!
(venv) ➜ Montserrat git:(merge-forked-changes)

```

(Note that also the message printed is wrong, as it assumes the fonts have been generated: `Done building fonts!`)

After trying different versions It worked with

```
(venv) ➜ Montserrat git:(merge-forked-changes) ✗ pip install gftools==0.9.48
[...]
Installing collected packages: gftools
Attempting uninstall: gftools
Found existing installation: gftools 0.9.51
Uninstalling gftools-0.9.51:
Successfully uninstalled gftools-0.9.51
Successfully installed gftools-0.9.48

(venv) ➜ Montserrat git:(merge-forked-changes) ✗ gftools builder sources/config.yaml
[1/137] fontmake -o ufo --instance-dir instance_ufo -g Montserrat.glyphs
INFO:fontmake.font_project:Building master UFOs and designspace from Glyphs source
INFO:glyphsLib.parser:Parsing .glyphs file
INFO:fontmake.font_project:Saving master_ufo/Montserrat.designspace
INFO:fontmake.font_project:Saving master_ufo/Montserrat-Thin.ufo
INFO:fontmake.font_project:Saving master_ufo/Montserrat-SemiBold.ufo
[...]
```

Everything works well :)

In these tests I started from an empty venv, installing only gftools.

The files are here in case you wanted to test: https://github.com/JulietaUla/Montserrat/tree/merge-forked-changes

Thanks