In order to run a Stratum v2 Template Provider as a "sidecar" via IPC, and to have bitcoin-node be reproducibly built, libmultiprocess needs to be built with guix.

Presumably this would happen as part of #10102 or #30437 eventually.

However if the TP "sidecar" is built on top of the Bitcoin Core codebase, like Sjors#48 attempts, then that binary should also be Guix built. So it makes sense to track this independently.

I made a naive attempt in Sjors@33d22b9 by simply adding MULTIPROCESS=1 to the depends build in contrib/guix/libexec/build.sh. That didn't work because curl is not accessible.

A workaround along those lines would be fine for Sjors#48 for the time being though.

cc @ryanofsky