pv-wrap: Try to share TMPDIR, etc. with the container
We share /tmp with the container, so it's consistent if we also share $TMPDIR and so on.
pv-wrap: Don't share temp directory below $XDG_RUNTIME_DIR
Snap sets TMPDIR=$XDG_RUNTIME_DIR/snap.steam, but its AppArmor profile won't allow us to bind-mount that path into our container.