trellis: Revert removal of CMAKE_INSTALL_DATADIR

This is used to refer to the database file. Without it
many tools can't be used.

Changed files
+2
pkgs
development
embedded
fpga
trellis
+2
pkgs/development/embedded/fpga/trellis/default.nix
···
nativeBuildInputs = [ cmake python3 ];
cmakeFlags = [
"-DCURRENT_GIT_VERSION=${realVersion}"
+
# TODO: should this be in stdenv instead?
+
"-DCMAKE_INSTALL_DATADIR=${placeholder "out"}/share"
];
preConfigure = ''