pacman 7.0.0 breaks pacstrap

Description:

The new feature, DownloadUser, introduced in pacman 7.0.0 breaks pacstrap script provided by this package.

Additional info:

• package version(s): pacman 7.0.0, arch-install-scripts 28
• config and/or log files: nil
• link to upstream bug report, if any: nil

Steps to reproduce:

*: All steps below are run with root user.

1. create a new directory for archlinux root: mkdir /var/lib/machines/arch
2. install archlinux with pacstrap: pacstrap -K -c /var/lib/machines/arch base
3. pacstrap failed, with outputs as below:

==> Creating install root at /var/lib/machines/arch
gpg: /var/lib/machines/arch/etc/pacman.d/gnupg/trustdb.gpg: trustdb created
gpg: no ultimately trusted keys found
gpg: starting migration from earlier GnuPG versions
gpg: porting secret keys from '/var/lib/machines/arch/etc/pacman.d/gnupg/secring.gpg' to gpg-agent
gpg: migration succeeded
==> Generating pacman master key. This may take some time.
gpg: Generating pacman keyring master key...
gpg: directory '/var/lib/machines/arch/etc/pacman.d/gnupg/openpgp-revocs.d' created
gpg: revocation certificate stored as '/var/lib/machines/arch/etc/pacman.d/gnupg/openpgp-revocs.d/5AFDD76D8EDF0B1E93E6FB7BAFB2D82281670119.rev'
gpg: Done
==> Updating trust database...
gpg: marginals needed: 3  completes needed: 1  trust model: pgp
gpg: depth: 0  valid:   1  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 1u
==> Installing packages to /var/lib/machines/arch
:: Synchronizing package databases...
error: could not open file /var/lib/machines/arch/var/lib/pacman/sync/download-06RfGS/core.db.part: Permission denied
error: failed to setup a download payload for core.db
error: failed to synchronize all databases (failed to retrieve some files)
==> ERROR: Failed to install packages to new root

Possible fix:

By commenting-out DownloadUser = alpm in /etc/pacman.conf on host, this issue could be addressed.

Seems it is caused by downloading files (e.g. core.db) from repository as a not-yet-exist user alpm?