This one was sneaky. `doom-plist-merge` was mutating the second plist
fed to it, causing issues upwind of its uses. In #7925, for example,
calling `doom-package-recipe` to read a package's recipe would end up
altering it, copying sub-properties of :recipe to other packages'
recipes.
If you've hit #7925, you'll also need to delete your build-cache to get
around the error. I.e.
rm -f $EMACSDIR/.local/straight/build-*-cache.el
doom sync # or upgrade
Fix: #7925