Change root to $SNAP

I want to change root to $SNAP dir to force application to use libraries from $SNAP/usr/lib dir. How to do it?

you don’t … the confinement does not allow chroot normally
… what you do is to change LD_LIBRARY_PATH instead …

for known paths like $SNAP/usr/lib or $SNAP/lib snapcraft magically does that for you, you do not need to do anything special to have youor app find the libs. if you want to add extra paths you extend the LD_LIBRARY_PATH variable in an environment: entry in your snapcraft.yaml