build: Add to mode of installed files, don't overwrite
build: Add to mode of installed files, don't overwrite

This was a bad translation from the Python.

chergert/gnome-continuous,djdeath/gnome-ostree,vrutkovs/vrutkovs.github.io,chergert/gnome-continuous,GNOME/gnome-ostree,vrutkovs/vrutkovs.github.io,djdeath/gnome-ostree,GNOME/gnome-ostree,GNOME/gnome-ostree,chergert/gnome-continuous,djdeath/gnome-ostree,vrutkovs/vrutkovs.github.io