Permission denied: '/etc/apt/sources.list.d/google-earth.list'

Thanks to the hint from Zanna I found the issue:

lsattr output showed that google-earth.list was set immutable.

~$ lsattr /etc/apt/sources.list.d/google-earth.list
----i--------e-- /etc/apt/sources.list.d/google-earth.list

With this attribute set, not even root can write the file.
To fix it, I just used chattr -i to remove the immutable attribute on the file:

~$ sudo chattr -i /etc/apt/sources.list.d/google-earth.list

I made that file immutable myself, because after each Google Earth Update, [arch=amd64] gets removed from the source file (blame Google !). I originally found that "immutable fix" on webupd8.