Always use FileSystemInterface::getTempDirectory() to get the temporary directory

Problem/Motivation

Right now, there's a discrepancy in the way Package Manager resolves the system temporary directory (where the staging area is created). In \Drupal\package_manager\ProcessFactory::getComposerHomePath(), we use FileSystemInterface::getTempDirectory(), and in \Drupal\package_manager\Stage::getStagingRoot() we use FileSystem::getOsTemporaryDirectory().

The difference is that the former will first try to use the file_temp_path setting before falling back to the system temporary directory, and the latter doesn't. This needs to be made consistent.

Steps to reproduce

Proposed resolution

Always use FileSystemInterface::getTempDirectory() to get the temporary file path.

Remaining tasks

Do it, and write tests.