Cleanup step in buildscript removes custom package added by package-list

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Describe the bug
I added a package I built myself to all the `package-list.*` files and it is installed successfully according to the build log. At the end of the build, the hooks are executed which (with the `--debug` flag to `lb build`) gives the following output:
```
D: Executing: /root/0010-disable-kexec-tools.hook.chroot
D: Executing: /root/0050-disable-sysvinit-tmpfs.hook.chroot
D: Executing: /root/999-cleanup-apt-cache.chroot
D: Executing: apt-get autoclean
```

The `999-cleanup-apt-cache.chroot` script is the last thing run that I can see in the repo, I wonder what executes the `apt-get autoclean` because that step removes my package again so the package is not included in the iso.

Is there a way to wither turn this cleanup step off or to prevent the removal of my package?
I already tried giving the `.deb` file the `Priority: required` property but that doesn't seem to work.
I also checked if my package has dependencies that are listed in `seeds/blacklist` and `platform/blacklist` but couldn't find any.

### To Reproduce
Steps to reproduce the behavior:
1. Add previously not installed package to all the `package-list.*` files
2. Start a build
3. Make sure the package is installed during the build
4. Wait for the hooks to run
5. Package is removed by `apt-get autoclean` step

### Expected behavior
The package should stay installed.

### Screenshots or screen recordings
Not applicable, I can provide the full build log on request.

### Logs
See above for relevant parts.

## Platform Information
- OS: elementary OS (Buildserver is running Debian Buster with some elementary packages needed for compilation of vala stuff, building using Docker)
- OS Version Juno
- Hardware info: N/A ("generic" dedicated server with Intel CPU)

- [x] I'm using the latest version from git that I've manually compiled
- [ ] ~~I'm using the latest released stable version~~

## Additional context
The package I'm trying to install contains some configuration files and references other packages I want to have installed by default. As stated above, I already checked if I depend on blacklisted packages but couldn't find any.