formula: add PIP_CACHE_DIR to build env

Library/Homebrew/cleanup.rb

@@ -30,6 +30,7 @@ module Homebrew
           glide_home
           java_cache
           npm_cache
+          pip_cache
           gclient_cache
         ].include?(pathname.basename.to_s)
       end

Library/Homebrew/formula.rb

@@ -2880,6 +2880,7 @@ class Formula
       GOPATH:                  "#{HOMEBREW_CACHE}/go_mod_cache",
       CARGO_HOME:              "#{HOMEBREW_CACHE}/cargo_cache",
       CURL_HOME:               ENV.fetch("CURL_HOME") { Dir.home },
+      PIP_CACHE_DIR:           "#{HOMEBREW_CACHE}/pip_cache",
       PYTHONDONTWRITEBYTECODE: "1",
     }
   end