Not all package managers set DESTDIR

In the build-aux/meson/postinstall.py included with the Meson template, there's this check, which is used to disable some commands that are intended not to be run in package manager builds:

# Package managers set this so we don't need to run
if not destdir:

But not all package managers set DESTDIR (for example, Nix doesn't). Could there be another way (a Meson option?) to opt into this?