net/gce.sh: Upgrade to Ubuntu 20.04

net/scripts/gce-provider.sh

@@ -174,10 +174,10 @@ cloud_CreateInstances() {
     # the stock Ubuntu 18.04 image and programmatically install CUDA after the
     # instance boots
     #
-    imageName="ubuntu-1804-bionic-v20181029-with-cuda-10-and-cuda-9-2 --image-project principal-lane-200702"
+    imageName="ubuntu-2004-focal-v20201211-with-cuda-10-2 --image-project principal-lane-200702"
   else
-    # Upstream Ubuntu 18.04 LTS image
-    imageName="ubuntu-1804-bionic-v20190813a --image-project ubuntu-os-cloud"
+    # Upstream Ubuntu 20.04 LTS image
+    imageName="ubuntu-2004-focal-v20201201 --image-project ubuntu-os-cloud"
   fi
 
   declare -a nodes

net/scripts/install-certbot.sh

@@ -4,9 +4,11 @@ set -ex
 [[ $(uname) = Linux ]] || exit 1
 [[ $USER = root ]] || exit 1
 
-apt-get update
-add-apt-repository --yes ppa:certbot/certbot
-apt-get --assume-yes install certbot
+snap install core
+snap refresh core
+
+snap install --classic certbot
+ln -sf /snap/bin/certbot /usr/bin/certbot
 
 cat > /certbot-restore.sh <<'EOF'
 #!/usr/bin/env bash