Fairly common occurences of this error reported in the past few days.
Typically, in the REST/HTTP world, GOAWAY is a hint for the client to backoff and close the stream as the server is likely backlogged (kind of like 5xx status codes, in a way). And so, retrying the flyctl command a few more times might help increase the chances of getting past that backpressure barrier.
Not sure why Fly’s build-instances are facing severe load of late, but I guess it is all the recent Heroku exodus to Render.comFly.io or their recent change to switch to do remote builds by default.