support downloading bottles through 3rd-party mirror

Closes Homebrew/homebrew#41328.

Closes Homebrew/homebrew#41332.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/cmd/config.rb

 require 'hardware'
+require "software_spec"
 
 module Homebrew
   def config
@@ -137,6 +138,7 @@ module Homebrew
     f.puts "Last commit: #{last_commit}"
     f.puts "HOMEBREW_PREFIX: #{HOMEBREW_PREFIX}"
     f.puts "HOMEBREW_CELLAR: #{HOMEBREW_CELLAR}"
+    f.puts "HOMEBREW_BOTTLE_DOMAIN: #{BottleSpecification::DEFAULT_DOMAIN}"
     f.puts hardware
     f.puts "OS X: #{MACOS_FULL_VERSION}-#{kernel}"
     f.puts "Xcode: #{xcode ? xcode : "N/A"}"

Library/Homebrew/manpages/brew.1.md

@@ -540,7 +540,10 @@ can take several different forms:
 
     Set this to force Homebrew to use a particular git binary.
 
-  * HOMEBREW_BROWSER:
+  * HOMEBREW\_BOTTLE\_DOMAIN:
+    If set, instructs Homebrew to use the given URL as a download mirror for bottles.
+
+  * HOMEBREW\_BROWSER:
     If set, uses this setting as the browser when opening project homepages,
     instead of the OS default browser.
 

Library/Homebrew/software_spec.rb

@@ -259,7 +259,7 @@ end
 class BottleSpecification
   DEFAULT_PREFIX = "/usr/local".freeze
   DEFAULT_CELLAR = "/usr/local/Cellar".freeze
-  DEFAULT_DOMAIN = "https://homebrew.bintray.com".freeze
+  DEFAULT_DOMAIN = (ENV["HOMEBREW_BOTTLE_DOMAIN"] || "https://homebrew.bintray.com").freeze
 
   attr_rw :prefix, :cellar, :revision
   attr_accessor :tap

share/man/man1/brew.1

@@ -507,6 +507,10 @@ When using Git, Homebrew will use \fBGIT\fR if set, a Homebrew\-built Git if ins
 Set this to force Homebrew to use a particular git binary\.
 .
 .TP
+HOMEBREW_BOTTLE_DOMAIN
+If set, instructs Homebrew to use the given URL as a download mirror for bottles\.
+.
+.TP
 HOMEBREW_BROWSER
 If set, uses this setting as the browser when opening project homepages, instead of the OS default browser\.
 .