apt.py will not attempt to install i386 packages on an amd64 system

# Bug report
On an amd64 systems with i386 architecture enabled, cloud-init will not install packages only available in i386

## Steps to reproduce the problem
Enable i386 arch in early boot before apt update:

bootcmd\:
\- [ cloud-init-per, once, dpkgi386, dpkg, --add-architecture, i386]

Add package available to apt in i386

packages:
\- steamcmd

cloud-init will not find the package to install. Package will install successfully from command line:

`apt install steamcmd`

Cause: cloud-init searches for packages using `apt-cache pkgnames` before attempting to install, which only lists packages for the primary architecture. Adding the `--all-names` flag to the call in `apt.py` does include packages for i386, but also includes virtual packages unavailable for installation.

## Environment details
- Cloud-init version: 25.1.4
- Operating System Distribution: Debian 13 Trixie
- Cloud provider, platform or installer type: vultr

## cloud-init logs
2026-09-07 17:24:37,859 - apt.py[DEBUG]: The following packages were not found by APT so APT will not attempt to install them: ['steamcmd']
2026-09-07 17:24:37,859 - distros[INFO]: Failed to install the following packages: ['steamcmd']. See associated package manager logs for more details.
2026-09-07 17:24:37,859 - log_util.py[WARNING]: Failure when attempting to install packages: ['steamcmd']
2026-09-07 17:24:37,860 - log_util.py[DEBUG]: Failure when attempting to install packages: ['steamcmd']
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/cloudinit/config/cc_package_update_upgrade_install.py", line 95, in handle
cloud.distro.install_packages(pkglist)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/distros/__init__.py", line 286, in install_packages
raise PackageInstallerError(error_message % total_failed)
cloudinit.distros.PackageInstallerError: Failed to install the following packages: {'steamcmd'}. See associated package manager logs for more details.
2026-09-07 17:24:37,863 - cc_package_update_upgrade_install.py[WARNING]: 1 failed with exceptions, re-raising the last one
2026-09-07 17:24:37,863 - handlers.py[DEBUG]: finish: modules-final/config-package-update-upgrade-install: FAIL: running config-package-update-upgrade-install with frequency once-per-instance
2026-09-07 17:24:37,863 - log_util.py[WARNING]: Running module package-update-upgrade-install () failed
2026-09-07 17:24:37,864 - log_util.py[DEBUG]: Running module package-update-upgrade-install () failed
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/cloudinit/config/modules.py", line 297, in _run_modules
ran, _r = cc.run(
~~~~~~^
run_name, mod.handle, func_args, freq=freq
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
)
^
File "/usr/lib/python3/dist-packages/cloudinit/cloud.py", line 71, in run
return self._runners.run(name, functor, args, freq, clear_on_fail)
~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/helpers.py", line 156, in run
results = functor(**args)
File "/usr/lib/python3/dist-packages/cloudinit/config/cc_package_update_upgrade_install.py", line 127, in handle
raise errors[-1]
File "/usr/lib/python3/dist-packages/cloudinit/config/cc_package_update_upgrade_install.py", line 95, in handle
cloud.distro.install_packages(pkglist)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/distros/__init__.py", line 286, in install_packages
raise PackageInstallerError(error_message % total_failed)
cloudinit.distros.PackageInstallerError: Failed to install the following packages: {'steamcmd'}. See associated package manager logs for more details.

Start in apt.py, focusing on the apt-cache pkgnames lookup used before package installation, and reproduce the report on Debian with i386 enabled before apt update. Verify that an i386-only package such as steamcmd is discovered and installed while virtual packages are not treated as installable.

Written by the indexing model from the issue text.