commands: ensure external commands are executable

For consistency with `brew command` and the logic in `brew.sh` (both use
`which` to find/validate an external command), we need to filter files
that are not executable.

Otherwise `brew commands` and thus bash completion will offer commands
that will produce an error when attempting to use them.

Closes Homebrew/homebrew#44999.

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

Library/Homebrew/cmd/commands.rb

@@ -38,6 +38,7 @@ module Homebrew
 
   def external_commands
     paths.flat_map { |p| Dir["#{p}/brew-*"] }.
+      select { |f| File.executable?(f) }.
       map { |f| File.basename(f, ".rb")[5..-1] }.
       reject { |f| f =~ /\./ }.
       sort