gnunet 0.21.2 build failure

[Rui Chen]

Hi,

:wave:, this is Rui, a homebrew maintainer, trying to upgrade gnunet
formula to the latest release, but failed to regression test the build
as the error shown below:

```
==> /opt/homebrew/Cellar/gnunet/0.21.2/bin/gnunet-config -s arm
2024-06-08T11:29:36.887623-0400 gnunet-config-79344 ERROR Unreadable
configuration file `(null)', exiting ...
```

relates to https://github.com/Homebrew/homebrew-core/pull/174035

Thanks,
Rui