Elide use of `ls`, `find` and `touch`

Elide use of `ls`, `find` and `touch`

File::Find and code copied from ExtUtils::Command do the same.

This patch makes the code to remove the exec bit a little less
efficient by not restricting itself to files that now have
the exec bit set, but instead looking at all files.

This change also uses $ENV{TEMP} in preference to
/tmp as a temp directory for caching the CPAN index.