New review comment by Johnnynator on void-packages repository

https://github.com/void-linux/void-packages/pull/28129#discussion_r567335991

Comment:
```suggestion
post)
if [ "$(readlink -f . 2>/dev/null || echo .)" != "/" ]; then
	exit 1
else
	mkdir -p var/lib/singularity/mnt/session
	chown root:_singularity usr/libexec/singularity/bin/starter-suid
	chmod 4755 usr/libexec/singularity/bin/starter-suid
fi
```
Maybe do smth like this to prevent changing the binary to a potentially wrong group when executing xbps-install with `--rootidr`