this should allow package tests that check things like writability to
succeed and have fewer skipped tests in CI.
- switch to using the `void-LIBC-full` container
- use `uchroot` instead of `ethereal`
- simplify some scripts for this change
The CI has `base-chroot` installed, but sometimes this script would try
to install `base-system,` causing conflicts. This commit fixes that
issue by using a brand-new rootdir.
