It is hardcoded, so your current options are: 1) download and use a boot floppy from Bell Labs[1] site, or 2) move your CD to another IDE cable (i.e. primary master on another cable) and go through the installation process again.
Seems like there would be an option number:3) Make a boot floppy, as discussed in update(8) and esp. the EXAMPLES subhead in prep(8).
[1] http://plan9.bell-labs.com/plan9/download/plan9.flp.gz
-- Josh
