cmd/upspin: use -context flag to locate context file
cmd/upspin: use -context flag to locate context file

Change-Id: I8808703942b30e3549f6438bc84906dc74e9c020
Reviewed-on: https://upspin-review.googlesource.com/4263
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

upspin/upspin,upspin/upspin