Fix mktmp to be generic as-per SUS/BSD.

Closes Homebrew/homebrew#31586.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/cmd/doctor.rb

@@ -700,7 +700,7 @@ def check_for_multiple_volumes
   # Find the volumes for the TMP folder & HOMEBREW_CELLAR
   real_cellar = HOMEBREW_CELLAR.realpath
 
-  tmp = Pathname.new with_system_path { `mktemp -d #{HOMEBREW_TEMP}/homebrew-brew-doctor-XXXX` }.strip
+  tmp = Pathname.new with_system_path { `mktemp -d #{HOMEBREW_TEMP}/homebrew-brew-doctor-XXXXXX` }.strip
   real_temp = tmp.realpath.parent
 
   where_cellar = volumes.which real_cellar

Library/Homebrew/extend/fileutils.rb

@@ -14,7 +14,7 @@ module FileUtils
     # /tmp volume to the other volume. So we let the user override the tmp
     # prefix if they need to.
 
-    tempd = with_system_path { `mktemp -d #{HOMEBREW_TEMP}/#{prefix}-XXXX` }.chuzzle
+    tempd = with_system_path { `mktemp -d #{HOMEBREW_TEMP}/#{prefix}-XXXXXX` }.chuzzle
     raise "Failed to create sandbox" if tempd.nil?
     prevd = pwd
     cd tempd