netbsd: take fetchcvs from buildPackages to fix eval

For some reason fetchcvs was taken from the cross package set when
cross compiling netbsd.* instead of from buildPackages. I suspect that
this was no issue previously because it just could be substituted as it
only ever produced fixed output derivations. However since
00a2084 the evaluation of fetchcvs
would depend on `stdenv.cc` (via its openssh dependency). Since
netbsd.libc is required to evaluate stdenv.cc this caused a infinite
recursion when staging the cross stdenv for pkgsCross.amd64-netbsd. The
fix for this is pretty easy: Just take fetchcvs from buildPackages
explicitly instead of hoping for mkDerivation to get it right.

Resolves NixOS#119323.

pkgs/os-specific/bsd/netbsd/default.nix

@@ -1,8 +1,8 @@
-{ stdenv, stdenvNoCC, fetchcvs, lib, groff, mandoc, zlib, bison, flex
+{ stdenv, stdenvNoCC, lib, groff, mandoc, zlib, bison, flex
 , writeText, buildPackages, splicePackages, symlinkJoin }:
 
 let
-  fetchNetBSD = path: version: sha256: fetchcvs {
+  fetchNetBSD = path: version: sha256: buildPackages.fetchcvs {
     cvsRoot = ":pserver:anoncvs@anoncvs.NetBSD.org:/cvsroot";
     module = "src/${path}";
     inherit sha256;