Meta: Download files at configure time a bit more atomically

Download files to a temporary location, then only move the downloaded
file to the real location once the download is complete. This prevents
CMake from being confused about partially-downloaded files, e.g. if
someone presses ctrl+c in the middle of a download.

Note the GN build already behaves this way.

Meta/CMake/utils.cmake

@@ -210,20 +210,23 @@ function(download_file_multisource urls path)
  if (NOT ENABLE_NETWORK_DOWNLOADS)
  message(FATAL_ERROR "${path} does not exist, and unable to download it")
  endif()
+
  get_filename_component(file "${path}" NAME)
+ set(tmp_path "${path}.tmp")
 
  foreach(url ${urls})
  message(STATUS "Downloading file ${file} from ${url}")
 
- file(DOWNLOAD "${url}" "${path}" INACTIVITY_TIMEOUT 10 STATUS download_result ${DOWNLOAD_SHA256})
+ file(DOWNLOAD "${url}" "${tmp_path}" INACTIVITY_TIMEOUT 10 STATUS download_result ${DOWNLOAD_SHA256})
  list(GET download_result 0 status_code)
  list(GET download_result 1 error_message)
 
  if (status_code EQUAL 0)
+ file(RENAME "${tmp_path}" "${path}")
  break()
  endif()
 
- file(REMOVE "${path}")
+ file(REMOVE "${tmp_path}")
  message(WARNING "Failed to download ${url}: ${error_message}")
  endforeach()