Make wrapper-attrs available on build.package

[MattSturgeon]

In #2874 we moved the wrapped nvim package from build.package to build.nvimPackage, so that build.package could be the symlinkJoin that includes the optional man-docs and nixvim-print-init.

This is a little clunky, in that package attrs previously available in build.package are no longer available here.

Two ways we could solve this include:

• not using symlinkJoin

We could bundle the extra bits directly into the package. E.g. using a postInstall hook.

This would mean changing enableMan would cause a full rebuild, but that's probably not a big deal for the wrapper package.

• Inherit important/useful attrs as passthru attrs on the symlinkJoin.

This may be simpler, but would of course only expose the attrs we explicitly inherit.