Fix singularity recipes to select Python version (default: 2)

Scripts/singularity/Singularity.egl

@@ -20,6 +20,7 @@ From: nvidia/opengl:1.0-glvnd-devel-ubuntu18.04
     SUPERBUILD_TAG=release
     SUPERBUILD_REPO=https://gitlab.kitware.com/paraview/paraview-superbuild.git
     DEV_BUILD=false
+    PYTHON_VERSION=2
 
     # First install some packages we need
     apt-get -y update
@@ -52,6 +53,7 @@ From: nvidia/opengl:1.0-glvnd-devel-ubuntu18.04
     # Make sure needed env vars are truly seen in the next step
     export PARAVIEW_TAG=${PARAVIEW_TAG}
     export RENDERING=${RENDERING}
+    export PYTHON_VERSION=${PYTHON_VERSION}
 
     # Configure/build/install ParaView
     /home/pv-user/cmake/3.13.4/bin/cmake -C /home/pv-user/pvsb/src/cmake/sites/Docker-Ubuntu-18_04.cmake "-GUnix Makefiles" ../src

Scripts/singularity/Singularity.osmesa

@@ -18,6 +18,7 @@ From: ubuntu:18.04
     SUPERBUILD_TAG=release
     SUPERBUILD_REPO=https://gitlab.kitware.com/paraview/paraview-superbuild.git
     DEV_BUILD=false
+    PYTHON_VERSION=2
 
     # First install some packages we need
     echo "\ndeb http://us.archive.ubuntu.com/ubuntu/ bionic universe\n" >> /etc/apt/sources.list
@@ -52,6 +53,7 @@ From: ubuntu:18.04
     # Make sure needed env vars are truly seen in the next step
     export PARAVIEW_TAG=${PARAVIEW_TAG}
     export RENDERING=${RENDERING}
+    export PYTHON_VERSION=${PYTHON_VERSION}
 
     # Configure/build/install ParaView
     /home/pv-user/cmake/3.13.4/bin/cmake -C /home/pv-user/pvsb/src/cmake/sites/Docker-Ubuntu-18_04.cmake "-GUnix Makefiles" ../src