don't apt-get from kbfs

Idk if this can or should be fixed, but trying to run `apt-get update` from a KBFS folder under Debian results in this error:

[...snip...]
Hit http://ftp.debian.org jessie/main amd64 Packages
Hit http://ftp.debian.org jessie/contrib amd64 Packages
Hit http://ftp.debian.org jessie/non-free amd64 Packages
Hit http://ftp.debian.org jessie/main i386 Packages
Hit http://ftp.debian.org jessie/contrib i386 Packages
Hit http://ftp.debian.org jessie/non-free i386 Packages
Hit http://ftp.debian.org jessie/contrib Translation-en
Hit http://ftp.debian.org jessie/main Translation-en
Hit http://ftp.debian.org jessie/non-free Translation-en
Fetched 105 kB in 3s (29.8 kB/s)
E: Unable to change to /keybase/ - chdir (13: Permission denied)

Reproduce the reported `apt-get update` failure while the working directory is under a KBFS folder on Debian. Start by determining whether the permission-denied `chdir` comes from KBFS or apt-get, then establish whether the behavior can be fixed or only documented; done means the failure is resolved or its limitation is clearly explained.

Written by the indexing model from the issue text.