cmd/upspin: search for config in $HOME/upspin if not found
cmd/upspin: search for config in $HOME/upspin if not found

Restore this functionality that was inadvertently removed with the
addition of the 'config' sub-command (commit bb72f3c77).

Change-Id: I1beee3ad3c6f9443f9f7414bbe6739c8ba180def
Reviewed-on: https://upspin-review.googlesource.com/15880
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin