This allows 'guix publish' threads as well as 'guix substitute' and
'guix offload' processes to be properly labeled in 'top', 'pstree', etc.
* guix/workers.scm (worker-thunk): Add #:thread-name parameter and honor it.
(make-pool): Likewise.
* guix/scripts/publish.scm (http-write): Add calls to 'set-thread-name'
in bodies of 'call-with-new-thread'.
(guix-publish): Call 'set-thread-name'. Pass #:thread-name to 'make-pool'.
* guix/scripts/offload.scm (guix-offload): Call 'set-thread-name'.
* guix/scripts/substitute.scm (guix-substitute): Likewise.
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