brew.rb: remove executable bit and shebang

Because `brew.rb` should never be invoked directly, remove executable
bit and shebang to make that more obvious.

Closes Homebrew/homebrew#48816.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

Library/brew.rb

-#!/System/Library/Frameworks/Ruby.framework/Versions/Current/usr/bin/ruby -W0
-
 std_trap = trap("INT") { exit! 130 } # no backtrace thanks
 
 require "pathname"