singularity-tools: create mount points for image building (#41470)

pkgs/build-support/singularity-tools/default.nix

···
             echo creating
             singularity image.create -s $((1 + size * 4 / 1024 + ${toString extraSpace})) $out
             echo importing
-            mkdir -p /var/singularity/mnt/container
+            mkdir -p /var/singularity/mnt/{container,final,overlay,session,source}
             tar -c . | singularity image.import $out
           '');