Fix test failures when build-time tests are run as root

In particular this happens when the CI pipeline runs reprotest.
6 jobs for wip/smcv/salsa-ci in 32 minutes and 9 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #207521
build

00:13:32

 
  Test
passed #207524
autopkgtest

00:06:08

passed #207525
blhc

00:01:37

passed #207523
lintian

00:02:11

passed #207526
piuparts

00:09:33

passed #207522
reprotest

00:18:37