bump-formula-pr: use GitHub vars under env filter.

Ensure that the various variables are passed through and reset
correctly. If we end up doing this in a bunch of places we may add some
helpers to simplify this.

Library/Homebrew/dev-cmd/bump-formula-pr.rb

@@ -117,6 +117,14 @@ module Homebrew
     # user path, too.
     ENV["PATH"] = ENV["HOMEBREW_PATH"]
 
+    # Setup GitHub environment variables
+    %w[GITHUB_USER GITHUB_PASSWORD GITHUB_TOKEN].each do |env|
+      homebrew_env = ENV["HOMEBREW_#{env}"]
+      next unless homebrew_env
+      next if homebrew_env.empty?
+      ENV[env] = homebrew_env
+    end
+
     formula = ARGV.formulae.first
 
     if formula

bin/brew

@@ -47,7 +47,8 @@ HOMEBREW_LIBRARY="$HOMEBREW_REPOSITORY/Library"
 # Whitelist and copy to HOMEBREW_* all variables previously mentioned in
 # manpage or used elsewhere by Homebrew.
 for VAR in AWS_ACCESS_KEY_ID AWS_SECRET_ACCESS_KEY BINTRAY_USER BINTRAY_KEY \
-           BROWSER EDITOR GIT PATH VISUAL
+           BROWSER EDITOR GIT PATH VISUAL \
+           GITHUB_USER GITHUB_PASSWORD GITHUB_TOKEN
 do
   # Skip if variable value is empty.
   [[ -z "${!VAR}" ]] && continue