Fixes <https://issues.guix.gnu.org/54723>.
Reported by Guillaume Le Vaillant <glv@posteo.net>.
Regression introduced in f743f2046b.
With commit f743f2046b, responses to
pipelined GETs would end up being written concurrently by many threads.
Thus the body of those responses could be interleaved and garbled.
* guix/scripts/publish.scm: Revert
f743f2046b.
* tests/publish.scm ("/*.narinfo pipeline"): New test.