Qemu binfmt on foreign distro

[Mathieu Othacehe <m.othacehe@gmail.com>]

Hello,

When running a guix build command with --system argument set to a
foreign architecture, guix-daemon will use linux binfmt mechanism to
emulate binary execution.

On a Guix system, the qemu-binfmt-service-type service has to be
installed, so that linux binfmt mechanism is hooked to QEMU. This
service also adds a bunch of "--chroot-directory" arguments to
guix-daemon command line, so that QEMU and all its dependencies are
accessible from the guix-daemon chroot.

On a foreign distro, one can install QEMU and setup the binfmt mechanism
(installing binfmt-support and qemu-user-static packages on
Ubuntu). However, without the proper --chroot-directory arguments passed
to guix-daemon, all binaries execution relying on binfmt will throw
ENOENT from within the guix-daemon.

So I can see two solutions here:

* Remove --system argument support for guix on foreign distributions
(except when using this argument does not require binfmt support, 32
bits build on a 64 bits machine).

* Provide some kind of script that generates the correct guix-daemon
command line, enabling binfmt support on foreign distros.

Anyhow, the current behaviour where guix build --system=... fails
misteriously because of an ENOENT error on a foreign distro is not
acceptable in my opinion.

What do you think?

Thanks,

Mathieu