In the install log, I get this:
WARNING: The directory '/usr/bin/false/.cache/pip' or its parent directory is not owned or is not writable by the current user. The cache has been disabled. Check the permissions and owner of that directory. If executing pip with sudo, you may want sudo's -H flag.
I'm installing with sudo (also used sudo -H but nothing changed) but also with root I get the same warning.
Are you running it as a user with shell
/usr/bin/false by any chance?
buildout is running as plone_buildout user. This is the installer default, I think. This users are created by the installer itself. There's no doc about how to run buildout in an Plone installer installation(testing or developing or production), other than "run buildout". Running as root rise a warning and stop. Running as plone_buildout lead to this warning. Files are chowned to plone_buildout.
There's also this:
maybe I should use a regular user added in this group but the cache would be local to that user.
I've found instructions in README.html. Plone 5.1 they are ok while Plone 5.2 they are wrong (missing the user, shoud be buildout_daemon):
<p>Adjust the ports Plone uses before starting the site, if necessary,
and run buildout to apply settings.</p>
<p>To start Plone, issue the following command in a terminal window:</p>
<pre>sudo -u /var/local/plone/zeocluster/bin/plonectl start</pre>
<p>To stop Plone, issue the following command in a terminal window:</p>
<pre>sudo -u /var/local/plone/zeocluster/bin/plonectl stop</pre>
I'm quite shocked about this missing/wron part.