Add Guix manifest

[ryanprior]

This PR adds guix.scm, providing a package for xc which uses the local source files in the git working tree instead of building from the last tagged release.

Outcomes for Guix users

Guix users working on xc will benefit in a variety of cases:

use case	example
provision a dev environment with all dependencies	guix shell --development
test whether the current git tree will build in Guix	guix build --file=guix.scm
check for undeclared runtime dependencies	guix shell --container --file=guix.scm -- xc

No documentation changes

This shouldn't have any impact on non-Guix users, and there's no documentation for Nix users presently, so I decided not to add any for this. If you would welcome it, I can write a paragraph in the README about using Guix for development.

Guix upstream

Upstream Guix accepted my patch to include the package for xc and its dependencies, which means we can ship a lean guix.scm.

[ryanprior]

Related: https://codeberg.org/guix/guix/pulls/9935

[joerdav]

Is it worthwhile updating any documentation for this one? Or is it more like "if you know, you know"

[ryanprior]

I'll just wing a paragraph here off the top of my head something like what I'd propose for the docs if we were to do them, in case it helps provoke a gut response as to whether it's valuable

Contributing to xc using Guix

An easy way to get started contributing to xc is to use GNU Guix, which has a package recipe for xc. Use guix environment --development with the included guix.scm to provision a dev environment. To check whether your current git working tree will build in an isolated environment, use guix build --file=guix.scm. This will only consider files which are tracked in git, so add any new files to your index first. To build & run from source for testing, use guix shell --file=guix.scm -- xc [args]. Once you push your git branch to a remote, other people can try your version of xc using guix shell --with-git-url=xc=https://github.com/your-username/xc --with-branch=xc=your-branch xc -- xc [args]

what do you think, add something like that to the docs, take a different approach, or skip?