[now-cli] Clean up builder dev server shutdown logic (#4109)
[now-cli] Clean up builder dev server shutdown logic (#4109)

DRY and removes the `pid` from the set immediately so there's no race
condition if the server is shut down shortly after an HTTP request
(like in the tests).
zeit/now-cli,zeit/now-cli,zeit/now-cli,zeit/now-cli,zeit/now-cli,zeit/now-cli,zeit/now-cli