This simplifies code and mostly ensures we don't print a spinner while
there's build activity going on.
* build-aux/build-self.scm (build-program): Remove 'spin' and
'call-with-new-thread' call from "compute-guix-derivation" body. Remove
"Computing Guix derivation" message.
(proxy): Pass extra argument to 'select'. Display a spinner when
'select' returns empty lists.
(build): Print "Computing Guix derivation" message here.
a81a19930b