Fix the script's module dictionary being printed to stderr

[TomiBelan]

Fixes #240.

For manual testing: see the example in #240.

$ shiv . -c helloworld -o out.pyz
[...]
$ ./out.pyz
Hello, world!
$ echo $?
0

Success! No stderr output and good return code!

I didn't add automated tests because this area of the shiv codebase has no coverage.