freetype: patch pkg-config in -config better
freetype: patch pkg-config in -config better

When we have build == host != target, we don't want to patch in a
prefixed pkg-config used for cross compiling. Using `pkgsHostHost`
expressses the intent.

Then again, per https://github.com/NixOS/nixpkgs/issues/51176 leaving
`buildPackages.pkg-config` is arguably also correct, if we do further
cross compilation and want to run `freetype-config`. Really, there is no
good solution.

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