Avoid mutating the script argument in place

Mutating the argument in place, and in particular deleting the
`:executable` entry, leads to a bug when the same code path leading to
read_script_arguments is invoked twice, like in
https://github.com/Homebrew/homebrew-cask/pull/139749

This commit makes a shallow copy of the argument, so that it can be
safely mutated in the rest of the method.

Library/Homebrew/cask/artifact/abstract_artifact.rb

@@ -99,7 +99,12 @@ module Cask
         description = key ? "#{stanza} #{key.inspect}" : stanza.to_s
 
         # backward-compatible string value
-        arguments = { executable: arguments } if arguments.is_a?(String)
+        if arguments.is_a?(String)
+          arguments = { executable: arguments }
+        else
+          # Avoid mutating the original argument
+          arguments = arguments.dup
+        end
 
         # key sanity
         permitted_keys = [:args, :input, :executable, :must_succeed, :sudo, :print_stdout, :print_stderr]