Improve HOMEBREW_TEMP handling

- if HOMEBREW_TEMP is not writable, use the default temp directory
- when running `brew bundle exec`, strip various temporary directories
  from the environment if they are not writable

Library/Homebrew/brew.sh

@@ -104,6 +104,10 @@ HOMEBREW_CASKROOM="${HOMEBREW_PREFIX}/Caskroom"
 HOMEBREW_CACHE="${HOMEBREW_CACHE:-${HOMEBREW_DEFAULT_CACHE}}"
 HOMEBREW_LOGS="${HOMEBREW_LOGS:-${HOMEBREW_DEFAULT_LOGS}}"
 HOMEBREW_TEMP="${HOMEBREW_TEMP:-${HOMEBREW_DEFAULT_TEMP}}"
+if [[ ! -w "${HOMEBREW_TEMP}" ]]
+then
+  HOMEBREW_TEMP="${HOMEBREW_DEFAULT_TEMP}"
+fi
 
 # commands that take a single or no arguments.
 # HOMEBREW_LIBRARY set by bin/brew

Library/Homebrew/bundle/commands/exec.rb

@@ -111,6 +111,14 @@ module Homebrew
           # For commands which aren't either absolute or relative
           raise "command was not found in your PATH: #{command}" if command.exclude?("/") && which(command).nil?
 
+          %w[HOMEBREW_TEMP TMPDIR HOMEBREW_TMPDIR].each do |var|
+            value = ENV.fetch(var, nil)
+            next if value.blank?
+            next if File.writable?(value)
+
+            ENV.delete(var)
+          end
+
           if subcommand == "env"
             ENV.sort.each do |key, value|
               # No need to export empty values.