plan9perl setup.rc script update

[ealvar3z]

Improves error handling, provides verbose output, and fixes
the early exit from the previous script which failed to copy
the build files to /sys/src/cmd/perl

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• This set of changes does not require a perldelta entry.

[jkeenan]

In our continuous integration test suite your pull request is encountering one failure: you need to add your name and email address to our AUTHORS file. You can do this by running perl Porting/updateAUTHORS.pl, following the prompts, committing the modified files and re-pushing your pull request.

Also, you should be advised that the changes in your pull request will only appear in a production version of Perl starting with 5.42 in May 2025. This will have no impact on older releases of Perl. (I'm not sure which version of Perl the Plan9 distribution currently provides.)