x/build/cmd/gomote: add a login command

[cagedmantis]

The gomote command transparently attempts to log in the user if a login credential doesn't exist. When a user's credentials have expired, the user has to remove the file storing the credentials manually. This has proven to be confusing and frustrating to gomote users. We should add a login command which forces a the creation of new authentication credentials.

[gopherbot]

Change https://go.dev/cl/601796 mentions this issue: cmd/gomote, internal/iapclient: add a gomote login command

[gopherbot]

Change https://go.dev/cl/615435 mentions this issue: internal/iapclient, cmd/gomote: handle authentication failures