pack/keygen: fix build
pack/keygen: fix build

To find a packer by name, use pack.LookupByName.

Change-Id: I1660f691c99a2555b384ad3e7c320b2b46378382
Reviewed-on: https://upspin-review.git.corp.google.com/1491
Reviewed-by: Eric Grosse <f6ae9ca1078fda92fd7c2c20411c29bc6711159e@google.com>

upspin/upspin,upspin/upspin