Bug#1084956: sbuild 0.86.0 breaks the netplan.io testsuite

[Aurelien Jarno <aurel32@debian.org>]

Package: sbuild
Version: 0.86.0
Severity: serious
Tags: ftbfs
X-Debbugs-Cc: team+networking@tracker.debian.org, debian-wb-team@lists.debian.org
Control: affects -1 + src:netplan.io

Dear maintainer,

sbuild 0.86.0 causes netplan.io to FTBFS with testsuite failure. A build
log is available there, but the problem is reproducible on amd64:
https://buildd.debian.org/status/fetch.php?pkg=netplan.io&arch=riscv64&ver=1.1-2&stamp=1728672152&raw=0

I have bisected the issue and this has been introduced by this commit:

commit e5a5dc7d258e939e62ad73f7b60f0b47507c6565
Author: Johannes Schauer Marin Rodrigues <josch@debian.org>
Date:   Tue Aug 27 20:24:46 2024 +0200

    refactor the unshare backend as /usr/libexec/sbuild-usernsexec
    
     - split out embedded perl into its own executable
     - an executable is forked anyways, because sbuild backends get exec-ed
       and then read from and written to stdout and stdin, respectively, but
       now that forked process is a normal perl program instead of embedded
       code
     - no more dumb-init neither inside nor outside the chroot
     - no more hundreds of lines of perl code in variables
     - no more dozens of lines of shell code inside perl variables
     - much less fork+exec
     - pivot root
     - src:dumb-init and src:python-psutil (the reason for dumb-init) still build
     - lib/Sbuild/Utility.pm: completely remove get_unshare_cmd()

Regards
Aurelien