bug#57933: Gtk is unsupported on i686-linux

[Mathieu Othacehe <othacehe@gnu.org>]

Hello,

I had the following error while trying out `make release` for
`i686-linux` specifically.

--8<---------------cut here---------------start------------->8---
+ for example in gnu/system/examples/*.tmpl
+ case "$example" in
+ options=
+ guix system -n disk-image gnu/system/examples/desktop.tmpl
accepted connection from pid 17139, user nixbld
guix system: warning: 'disk-image' is deprecated: use 'image' instead
guix system: error: package gvfs@1.50.2 does not support i686-linux
+ rm -f t-guix-system-16656 t-guix-system-error-16656 /tmp/guix-build-guix-1.3.0.24760-34049.drv-0/t-guix-system-16656/config.scm /tmp/guix-build-guix-1.3.0.24760-34049.drv-0/t-guix-system-16656/my-torrc
+ rmdir /tmp/guix-build-guix-1.3.0.24760-34049.drv-0/t-guix-system-16656
FAIL tests/guix-system.sh (exit status: 1)
--8<---------------cut here---------------end--------------->8---

Turns out, Gtk is unsupported on i686-linux which is problematic to
build the desktop.tmpl image.

--8<---------------cut here---------------start------------->8---
mathieu@meije ~/guix [env]$ make -j8 && ./pre-inst-env guix build gtk -s i686-linux
gnu/packages/gtk.scm:1182:2: warning: package gtk@4.8.0 does not support i686-linux
--8<---------------cut here---------------end--------------->8---

That's because `gst-plugins-bad` and `librsvg-bootstrap` both refer to
`librsvg` which depends on Rust which is only supported on x86_64-linux.

There are other packages in (gnu packages gnome) relying on librsvg
directly. I'm not sure what's our best option here. Use
librsvg-for-system for the entire desktop.tmpl closure?

Thanks,

Mathieu