setupwriters assumes upspin/deploy/ directory structure that setup wizard doesn't create

[alltom]

In order to successfully invoke upspin setupwriters -domain myhostname, I needed to create ~/upspin/deploy/myhostname/ and copy ~/upspin/config.upspinserver to ~/upspin/deploy/myhostname/config.

I kinda just guessed to do that based on the error messages and upspin setupwriters -help.

It seems that either setupwriters should take a -config flag like upspin, or the setup wizard should initialize the ~/upspin/deploy/ directory structure when it sets the server up on GCP.