strace: assert isLinux to avoid eval error on darwin (#114974)

libunwind.supportsHost is not available on darwin because it uses a different libunwind package (https://github.com/NixOS/nixpkgs/blob/master/pkgs/os-specific/darwin/apple-source-releases/default.nix#L270) and changing the stdenv is a big overkill.

pkgs/development/tools/misc/strace/default.nix

@@ -1,5 +1,9 @@
 { lib, stdenv, fetchurl, perl, libunwind, buildPackages }:
 
+# libunwind does not have the supportsHost attribute on darwin, thus
+# when this package is evaluated it causes an evaluation error
+assert stdenv.isLinux;
+
 stdenv.mkDerivation rec {
   pname = "strace";
   version = "5.11";