publish: Send uncached narinfo replies from the main thread.

publish: Send uncached narinfo replies from the main thread.

Fixes <https://issues.guix.gnu.org/54723>.
Reported by Guillaume Le Vaillant <glv@posteo.net>.

Regression introduced in f743f2046be2c5a338ab871ae8666d8f6de7440b.

With commit f743f2046be2c5a338ab871ae8666d8f6de7440b, 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
f743f2046be2c5a338ab871ae8666d8f6de7440b.
* tests/publish.scm ("/*.narinfo pipeline"): New test.