Fix annoying apt-get update error

[fagiani]

There is a message that is persistent on standard error output that won't fail the apt-get update command by is quite annoying and often misleads users in case of a real error causing unnecessary confusion:

===> BUILDING
[apt]
-----> Reusing cache
-----> Updating apt caches
       Hit:1 http://archive.ubuntu.com/ubuntu focal InRelease
       Get:2 http://apt.postgresql.org/pub/repos/apt focal-pgdg InRelease [91.7 kB]
       Hit:3 http://archive.ubuntu.com/ubuntu focal-security InRelease
       Hit:4 http://archive.ubuntu.com/ubuntu focal-updates InRelease
       Get:5 http://apt.postgresql.org/pub/repos/apt focal-pgdg/main amd64 Packages [390 kB]
rm: cannot remove '/var/cache/apt/archives/partial/*.deb': Permission denied  <--------- This message
       Fetched 482 kB in 1s (592 kB/s)
       Reading package lists...

Grepping out with a subset of this message within stderr should hide it without compromising other eventual errors.