Test a lot of builds

[rsheeter]

@simoncozens has been doing work on automating builds across the collection that we can exploit to build and ttx_diff a lot of fonts. "build me a VF" is an operation (implemented here: https://github.com/googlefonts/gftools/blob/builder2/Lib/gftools/builder/operations/buildVariable.py). Replace that line with a fontc equivalent, and off we go.

This should enable us to test "all the fonts for which we have a known upstream repository and where there's a gftools-bulider configuration file in that repo." At time of writing that means 482 fonts out of 1603.

TODO: add end to end instructions to acquire and build as many google fonts as possible.

[simoncozens]

Here is how I would do this:

• https://github.com/simoncozens/gf-dashboard/blob/main/gfautobuilder.py is a driver script which clones each repo with a known upstream and runs gftools-builder on it.
  • It requires a cache file with information about which upstream repos seem like they can be built using gftools-builder.
  • You can download the cache file directly or you can generate it yourself with this script.
    • If you choose to generate the cache file yourself, the list-known-upstreams.py script in turn requires the googlefonts Python module in the same repo, it requires you to set GFDIR on line 13 to your google/fonts checkout directory, and also requires you to set the environment variable GITHUB_TOKEN to a valid authentication token.
• Next you will need a version of gftools-builder which uses fontc instead of fontmake.
  • By the time you act on this, builder2 should be merged into the main branch, but in case it's not, start by checking out the builder2 branch of gftools.
  • Edit the files Lib/gftools/builder/operations/buildVariable.py, Lib/gftools/builder/operations/buildTTF.py and Lib/gftools/builder/operations/buildOTF.py, replacing the fontmake command line with the equivalent fontc command line. (The $fontmake_type variable means -g for Glyphs source, -m for designspace and -u for ufo; you don't need that font fontc.)
  • Install this modified gftools-builder into your Python environment; you now have a fontc-powered gftools-builder
• Run gfautobuilder.py and watch the pretty error messages.

[rsheeter]

Based on the comments in #852 I suggest we create a standalone (as in not part of the fontc repo) Rust tool that can do tricks like enumerate sources and stop using https://github.com/googlefonts/gf-dashboard/blob/main/list-known-upstreams.py. That would let us rig a nice "build all known GF sources" for fontc QA.