Tweak platform conditional in patch to support cross builds (#2816)

Author: iskunk

patches/core/ungoogled-chromium/disable-domain-reliability.patch

@@ -86,7 +86,7 @@
 -    "--output",
 -    rebase_path(output_file, root_build_dir),
 -  ]
-+  if (is_win) {
++  if (host_os == "win") {
 +      args = [
 +        "--file-list",
 +        "nul",