Un-escape \n in DEVELOPER_KEY_CONTENT before openssl conversion

GitHub secrets store multi-line PEMs as a single line with literal "\n"
sequences. Fastlane's app_store_connect_api_key action un-escapes them
via gsub before use; the helper must do the same before writing the
temp file, otherwise openssl sees garbage.

fastlane/Fastfile

@@ -33,6 +33,10 @@ def developer_key_content
   return @developer_key_content if defined?(@developer_key_content)
   content = ENV['DEVELOPER_KEY_CONTENT']
   return @developer_key_content = nil if content.nil? || content.empty?
+  # GitHub secrets carry multi-line PEMs as a single line with literal "\n"
+  # escapes; fastlane's action un-escapes these before use, so do the same
+  # here before invoking openssl.
+  content = content.gsub('\n', "\n")
   @developer_key_content = Tempfile.open(['AuthKey', '.p8']) do |f|
     f.write(content)
     f.flush