Thanks to Efraim for pointing out that we could simply switch Shepherd
to refer to guile-3.0 instead of guile-3.0-latest.  The attached patch
does this.

Now updating guile-3.0-latest causes much less churn:

Building the following 23 packages would ensure 47 dependent packages
are rebuilt: guile-studio@0.1.1-1.dd0ad42 guile-chickadee@0.10.0
guile-gemini@0.1 guile-openai@0.2-1.751cd5d guile-newra@0-0.266e72e
haunt@0.3.0 guile-bash@0.1.6-0.1eabc56 lokke@0.0.0-1.92d3637
swineherd@0.0.4 cuirass@1.2.0-6.0eaf7b6 emacs-guix@0.5.2-7.455272c
guile-imanifest@0.0.0-0.ccd5a21 cl-ospm@0.0.2 guix-jupyter@0.2.2
guix-build-coordinator-agent-only@0-109.406db8a
nar-herder@0-37.82f9371 guix-minimal@1.4.0-23.843b85c gwl@0.5.1
gwl-next@0.5.0-1.706a089 guix-modules@0.1.0
guix-daemon@1.4.0-23.843b85c bffe@0-6.7df2aa6 hpcguix-web@0.4.1

- Dave