The command at the top of this file fails to evaluate:
```
$ nix-build -A tests.pkg-config.defaultPkgConfigPackages
in job ‘nixpkgs.tests.pkg-config.defaultPkgConfigPackages.tests-combined.x86_64-linux’:
error: pkg-config module `recurseForDerivations` is not defined to be a derivation. Please check the attribute value for `recurseForDerivations` in `pkgs/top-level/pkg-config-packages.nix` in Nixpkgs.
```
This is also causing eval errors on Hydra:
https://hydra.nixos.org/jobset/nixos/pr-209870-gcc-external-bootstrap#tabs-errors
Let's filter out `recurseForDerivations=true` from the attrset,
since it exists mainly as a flag to signal special handling when
recursing.
