Change default temp on Linux to /var/tmp.

Previously, the default temporary directory was /tmp on Linux and
/private/tmp on macOS. On many Linux distros, including at least Fedora,
/tmp is stored in RAM. This diverges from the behavior on macOS and has
led to bugs, most notably the inability to install large bottles on
memory-limited machines.

This fixes Homebrew#19037.

Library/Homebrew/brew.sh

@@ -65,7 +65,7 @@ else
   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"
+  HOMEBREW_DEFAULT_TEMP="/var/tmp"
 fi
 
 realpath() {

Library/Homebrew/env_config.rb

@@ -457,7 +457,7 @@ module EnvConfig
                       "different volumes, as macOS has trouble moving symlinks across volumes when the target " \
                       "does not yet exist. This issue typically occurs when using FileVault or custom SSD " \
                       "configurations.",
-        default_text: "macOS: `/private/tmp`, Linux: `/tmp`.",
+        default_text: "macOS: `/private/tmp`, Linux: `/var/tmp`.",
         default:      HOMEBREW_DEFAULT_TEMP,
       },
       HOMEBREW_UPDATE_TO_TAG:                    {