So the risk is when installing into a shared prefix where someone else already created the cache, like `/usr/local`? If the cache does not exist (e.g. when installing into a Geany-specific prefix) it'll be fine because GTK will do a full lookup, right?
Given that, and that autotools is still there and this generally does not affect packages (who set DESTDIR and must a find different solution anyway), I think I would go @eli-schwartz suggestion in https://github.com/geany/geany/pull/2761#issuecomment-1046137818 except perhaps printing the exact command that's needed. With time meson 0.57 will propagate to the systems. If that's not sufficient we can find a solution later on.