Having just swatted away this bug while installing bullseye on a laptop that has 1xUSB3, 2xUSB-C and no ethernet, I thought I should share the workaround here. This is only necessary, I'm led to believe, when installing by WiFi without the installation of a Desktop.
Before tearing down the WiFi interface at the very end of the installation, the d-i should simply copy the existing /target/etc/network/interfaces file to /target/root/, perhaps named as etc-network-interfaces-at-installation, with its permissions set to 600. When the machine is rebooted, it is now straightforward for the sysadmin to login and simply: # mv /root/etc-network-interfaces-at-installation /etc/network/interfaces # ifup w<TAB><RETURN> to give themselves WiFi connectivity (using sudo if necessary). This workaround has the benefit of being fail-safe: the file has no effect on the system if the sysadmin ignores it and leaves it in /root, but if they do move it as above, then they've taken responsibility for its effects on any software they install later. Naturally, it would require a note in the Installation Guide for this method to be useful if the sysadmin is not one to make use of root's home directory. Cheers, David.
