Pushed a fix (?) for ACL key location

[Christopher Lemmer Webber]

Commit 6680880f9b8dceb4f2f3f91bd2b13c659b53835e pushed out a new version
of Guix, and it looks like it wasn't possible to build new systems from
that because the filename for the "Berlin ACL key" changed.  (Or at
least, I couldn't run "guix system vm".)

I pushed out a "fix" for this.  I hope it's ok.

I'm presuming this was expected to change given
ec4c404c0575b340a04b3922fb828ac5417832dd appearing to try to compensate
in the other direction *before* the new "guix" package version was
pushed.

If I made things worse, sorry in advance.  It fixes things here, but I
don't know the full context of what was going on.

 - Chris