treewide/nixos: use stdenv.cc.libc instead of glibc when available

This prevents duplication in cross-compiled nixos machines. The
bootstrapped glibc differs from the natively compiled one, so we get
two glibc’s in the closure. To reduce closure size, just use
stdenv.cc.libc where available.

nixos/modules/config/networking.nix

@@ -197,7 +197,7 @@ in
 
       } // optionalAttrs (pkgs.stdenv.hostPlatform.libc == "glibc") {
         # /etc/rpc: RPC program numbers.
-        rpc.source = pkgs.glibc.out + "/etc/rpc";
+        rpc.source = pkgs.stdenv.cc.libc.out + "/etc/rpc";
       };
 
       networking.proxy.envVars =