This brings cmake inline with the behaviour used for configure
scripts, defined in multiple-outputs.sh. It's important because
that setup hook will only set shareDocName if multiple outputs are in
use (and setOutputFlags hasn't been disabled). So previously,
CMAKE_INSTALL_DOCDIR would be set to $out/share/doc for single-output
derivations, instead of $out/share/doc/$shareDocName, which would
result in collisions.
Since this hook now uses the setOutputFlags variable, I had to remove
the empty assignment of it added in
a714284d8b.
Fixes: https://github.com/NixOS/nixpkgs/issues/82304
be1b22538a