When subprocesses are started with ProcessBuilder, they're forked by the
java process directly rather than from a shell, which can be surprising
for our use case here in the packaging tests which is similar to
scripting.

This commit changes the tests to run their subprocess commands in a
shell, using the bash -c <script> syntax for commands on linux and using
the powershell.exe -Command <script> syntax for commands on windows.
This syntax on windows is essentially what the tests were already doing.