singularity: 2.5.1 -> 2.5.2

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/singularity/versions.

<details><summary>Version release notes (from GitHub)</summary>
Greetings Singularity containerizers!

This release contains fixes for a _high severity_ security issue affecting Singularity 2.3.0 through 2.5.1 on kernels that support overlay file systems (CVE-2018-12021). A malicious user with network access to the host system (e.g. ssh) could exploit this vulnerability to access sensitive information on disk and bypass directory image restrictions like those preventing the root file system from being mounted into the container.

Singularity 2.5.2 should be installed immediately, and all previous versions of Singularity should be removed. The vulnerability addressed in this release affects kernels that support overlayfs. If you are unable to upgrade immediately, you should set `enable overlay =...