Debian update of Keybase fails bc of mount point

I wanted to update Keybase from 5.1.x to 5.2.0-20200130 on Debian 9.11 (x86_64) via the repository. It throws an error during update in aptitude:

```
Performing actions...
Setting up keybase (5.2.0-20200130211428.cf82db8320) ...
Autorestarting Keybase via systemd for USER.
mkdir: cannot create directory ‘/keybase’: File exists
chown: cannot access '/keybase': Transport endpoint is not connected
chmod: cannot access '/keybase': Transport endpoint is not connected
dpkg: error processing package keybase (--configure):
subprocess installed post-installation script returned error exit status 1
su: No module specific data is present
su: No module specific data is present
su: No module specific data is present
Errors were encountered while processing:
keybase
needrestart is being skipped since dpkg has failed
E: Sub-process /usr/bin/dpkg returned an error code (1)
Setting up keybase (5.2.0-20200130211428.cf82db8320) ...
Autorestarting Keybase via systemd for USER.
mkdir: cannot create directory ‘/keybase’: File exists
chown: cannot access '/keybase': Transport endpoint is not connected
chmod: cannot access '/keybase': Transport endpoint is not connected
dpkg: error processing package keybase (--configure):
subprocess installed post-installation script returned error exit status 1
su: No module specific data is present
su: No module specific data is present
su: No module specific data is present
Errors were encountered while processing:
keybase
Press Return to continue, 'q' followed by Return to quit.
```

Start by reproducing the aptitude update on Debian 9.11 and inspect the Keybase post-installation step that reports the /keybase mount errors. Done means the package configures successfully and the /keybase mount remains usable after the update.

Written by the indexing model from the issue text.