HOMEBREW_CURL_ARGS: add -R to preserve remote timestamp

Reference from `man curl`

>  -R, --remote-time
>    When  used,  this will make curl attempt to figure out the time-
>    stamp of the remote file, and if  that  is  available  make  the
>    local file get that same timestamp.

Library/Homebrew/global.rb

@@ -30,7 +30,7 @@ RUBY_BIN = RUBY_PATH.dirname
 HOMEBREW_USER_AGENT_CURL = ENV["HOMEBREW_USER_AGENT_CURL"]
 HOMEBREW_USER_AGENT_RUBY = "#{ENV["HOMEBREW_USER_AGENT"]} ruby/#{RUBY_VERSION}-p#{RUBY_PATCHLEVEL}"
 
-HOMEBREW_CURL_ARGS = "-f#LA"
+HOMEBREW_CURL_ARGS = "-f#RLA"
 
 require "tap_constants"