as to why, coverage in this diff D8636 is not showing up properly:

After building the images locally:

extract friday evening:
```
18:55 <ardumont> jenkins@d3bc8d7c77ae:~$ which nix-store
18:55 <ardumont> /nix/var/nix/profiles/default/bin/nix-store
18:55 <ardumont> jenkins@d3bc8d7c77ae:~$ nix-store --dump /nix/var/nix/profiles | sha256sum
18:55 <ardumont> 1789826dad0944312df4937d07e1215c9cc02128df06882fdf66e3786f4c4c6e  -
```

extract from now (same builds):
```
$ docker run -ti --name test-swh-tox --entrypoint bash swh-jenkins/tox
jenkins@1370971fc780:~$ which nix-store
/nix/var/nix/profiles/default/bin/nix-store
jenkins@1370971fc780:~$ env | grep PATH
PATH=/home/jenkins/.nix-profile/bin:/nix/var/nix/profiles/default/bin:/usr/local/openjdk-11/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/opt/emsdk/upstream/emscripten:/home/jenkins/.local/bin
```

Remote extract on the jenkins node running ci builds:
```
root@thyssen:~# docker run -ti --name test-swh-tox --entrypoint bash swh-jenkins/tox
jenkins@c56bc9324446:~$ which nix-store
/nix/var/nix/profiles/default/bin/nix-store
jenkins@c56bc9324446:~$ env | grep PATH
PATH=/home/jenkins/.nix-profile/bin:/nix/var/nix/profiles/default/bin:/usr/local/openjdk-11/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/opt/emsdk/upstream/emscripten:/home/jenkins/.local/bin
jenkins@c56bc9324446:~$ ls -lah
total 40K
drwxr-xr-x 1 jenkins jenkins 4.0K Oct  7 15:50 .
drwxr-xr-x 1 root    root    4.0K Oct  7 15:49 ..
-rw-r--r-- 1 jenkins jenkins  220 Apr 18  2019 .bash_logout
-rw-r--r-- 1 jenkins jenkins 3.5K Apr 18  2019 .bashrc
drwxr-xr-x 3 jenkins jenkins 4.0K Oct  7 15:50 .cache
drwxr-xr-x 2 jenkins jenkins 4.0K Oct  7 15:49 .jenkins
drwx------ 5 jenkins jenkins 4.0K Oct  7 15:50 .local
-rw-r--r-- 1 jenkins jenkins  807 Apr 18  2019 .profile
drwxr-xr-x 2 jenkins jenkins 4.0K Oct  7 15:49 agent
jenkins@c56bc9324446:~$ nix-store --dump . | sha256sum
8831d230ead221b2ba9835ae832d6ec467e194059c2a4c264e4d9b88ed14b0e8  -
```