The `src.meta` attribute does not exist, and even if it did the derivation does
not have a `rec` to reference it. This appears to be breaking all ofBorg meta
evaluation checks:
```
while evaluating the attribute 'homepage' at /var/lib/ofborg/checkout/repo/38dca4e3aa6bca43ea96d2fcc04e8229/mr-est/eval-0-gleber.ewr1.nix.ci/pkgs/misc/lilypond/fonts.nix:34:27:
undefined variable 'src' at /var/lib/ofborg/checkout/repo/38dca4e3aa6bca43ea96d2fcc04e8229/mr-est/eval-0-gleber.ewr1.nix.ci/pkgs/misc/lilypond/fonts.nix:34:18
```
https://gist.github.com/GrahamcOfBorg/cd87009bfee1a9c0d4f0d828b82ee09a
In development, I experimented with file globs, for loops and making the
supplementary fonts optional. As such, the original PR ended up with a
dysfunctional combination.
This commit fully adopts the for loop approach, which I prefer, since failing
matches, e.g. missing supplementary-fonts/, won't cause a failure.
