brew.rb: fix help regex.

Don't match `help` at the end of every string because `brew installhelp`
shouldn't really be running help.

Closes Homebrew/homebrew#36559.
Closes Homebrew/homebrew#36576.

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

Library/brew.rb

@@ -82,7 +82,7 @@ begin
              }
 
   empty_argv = ARGV.empty?
-  help_regex = /(-h$|--help$|--usage$|-\?$|help$)/
+  help_regex = /(-h$|--help$|--usage$|-\?$|^help$)/
   help_flag = false
   cmd = nil