gems is trying to access /usr/bin

[Tim Olsen <tim@brooklynpenguin.com>]

I have libgems-ruby1.8 1.0.1-4 installed.  I setgid the /var/lib/gems
directory with 775 permissions and owned by group gems.  This way I
can install gems without being root (I only need to belong to the group
gems).

This has the side effect of not being able to touch /usr/bin, which
I've noticed ruby gems has been trying to do lately:

tolsen@neurofunk:/var/lib$ gem install builder testunitxml
shared-mime-info rcov flexmock Bulk updating Gem source index for:
http://gems.rubyforge.org Successfully installed builder-2.1.2
Successfully installed testunitxml-0.1.5
Successfully installed shared-mime-info-0.1
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions into the /usr/bin directory.
tolsen@neurofunk:/var/lib$

That is with a fresh rubygems install (purged the packages,
deleted /var/lib/gems, installed rubygems again, changed permissions
as described above).

Any idea why rubygems is trying to touch /usr/bin ?  Has anyone else
noticed this?

Thanks,
Tim