user XDG_CACHE_HOME on Linux

This variable was getting filtered out before so it wasn't possible
for us to use it as the default for HOMEBREW_DEFAULT_CACHE
and HOMEBREW_DEFAULT_LOGS.

Library/Homebrew/brew.sh

@@ -38,7 +38,7 @@ then
   HOMEBREW_DEFAULT_LOGS="${HOME}/Library/Logs/Homebrew"
   HOMEBREW_DEFAULT_TEMP="/private/tmp"
 else
-  CACHE_HOME="${XDG_CACHE_HOME:-${HOME}/.cache}"
+  CACHE_HOME="${HOMEBREW_XDG_CACHE_HOME:-${HOME}/.cache}"
   HOMEBREW_DEFAULT_CACHE="${CACHE_HOME}/Homebrew"
   HOMEBREW_DEFAULT_LOGS="${CACHE_HOME}/Homebrew/Logs"
   HOMEBREW_DEFAULT_TEMP="/tmp"

bin/brew

@@ -185,6 +185,13 @@ then
   export HOMEBREW_EDITOR="${VISUAL}"
 fi
 
+# set from user environment
+# Needed to create the default cache and log paths on Linux.
+if [[ -n "${XDG_CACHE_HOME:-}" ]]
+then
+  export HOMEBREW_XDG_CACHE_HOME="${XDG_CACHE_HOME}"
+fi
+
 # set from user environment
 # shellcheck disable=SC2154
 # Set CI variable for Azure Pipelines and Jenkins