Don't download hermes nightly tarball if it exists (#36368)

Summary:
Currently the hermes tarball will be re-downloaded every time we run pod update even if the file already exists. This adds a check to avoid downloading it if it already exists. To avoid partial downloads causing issues (if the script is interrupted) we download first to a different file and rename it when done.

## Changelog

[IOS] [FIXED] - Don't download hermes nightly tarball if it exists

Pull Request resolved: #36368

Test Plan: Tested in an app using nightly builds that the download is only done once when running multiple pod update.

Reviewed By: sshic

Differential Revision: D43815772

Pulled By: cipolleschi

fbshipit-source-id: 9dedb79cab1f1cb37cd82b425d93515c6e1728c1

sdks/hermes-engine/hermes-utils.rb

@@ -110,9 +110,13 @@ def download_nightly_hermes(react_native_path, version)
     tarball_url = nightly_tarball_url(version)
 
     destination_folder = "#{react_native_path}/sdks/downloads"
-    destination_path = "#{destination_folder}/hermes-ios.tar.gz"
+    destination_path = "#{destination_folder}/hermes-ios-#{version}.tar.gz"
 
-    `mkdir -p "#{destination_folder}" && curl "#{tarball_url}" -Lo "#{destination_path}"`
+    unless File.exist?(destination_path)
+      # Download to a temporary file first so we don't cache incomplete downloads.
+      tmp_file = "#{destination_folder}/hermes-ios.download"
+      `mkdir -p "#{destination_folder}" && curl "#{tarball_url}" -Lo "#{tmp_file}" && mv "#{tmp_file}" "#{destination_path}"`
+    end
     return destination_path
 end