Ubuntu/Debian upgrades fail: ln: failed to create symbolic link ‘/usr/local/bin/drush’: File exists

I had to:
sudo mv /usr/local/bin/drush /usr/local/bin/drush.bak
then
sudo apt-get upgrade

Reproduce:
on Ubuntu

sudo apt-get upgrade

the aegir upgrade fails:

Setting up aegir3-provision (3.0~beta1+55.b0b0d65) ...
#!/usr/bin/env php
All settings correct for using Composer
Downloading...

Composer successfully installed to: //composer.phar
Use it: php composer.phar
Changed current directory to /var/aegir/.composer
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
- Removing drush/drush (6.5.0)
- Installing drush/drush (6.6.0)
Downloading: 100%

Writing lock file
Generating autoload files
ln: failed to create symbolic link ‘/usr/local/bin/drush’: File exists
dpkg: error processing package aegir3-provision (--configure):
subprocess installed post-installation script returned error exit status 1
dpkg: dependency problems prevent configuration of aegir3-hostmaster:
aegir3-hostmaster depends on aegir3-provision (>= 3.0~beta1+55.b0b0d65); however:
Package aegir3-provision is not configured yet.

dpkg: error processing package aegir3-hostmaster (--configure):
dependency problems - leaving unconfigured
dpkg: dependency problems prevent configuration of aegir3:
aegir3 depends on aegir3-provision (>= 3.0~beta1+55.b0b0d65); however:
Package aegir3-provision is not configured yet.
aegir3 depends on aegir3-hostmaster (>= 3.0~beta1+55.b0b0d65); however:
Package aegir3-hostmaster is not configured yet.

dpkg: error processing package aegir3 (--configure):
dependency problems - leaving unconfigured
No apport report written because the error message indicates its a followup error from a previous failure.
No apport report written because the error message indicates its a followup error from a previous failure.
Errors were encountered while processing:
aegir3-provision
aegir3-hostmaster
aegir3