gce/fetch-ssh-keys: Put script in separate file, use PrivateTmp...

...check the script with shfmt and shellcheck + some other minor
refactoring.

nixos/modules/virtualisation/fetch-instance-ssh-keys.bash

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+WGET() {
+    wget --retry-connrefused -t 15 --waitretry=10 --header='Metadata-Flavor: Google' "$@"
+}
+
+# When dealing with cryptographic keys, we want to keep things private.
+umask 077
+mkdir -p /root/.ssh
+
+echo "Fetching authorized keys..."
+WGET -O /tmp/auth_keys http://metadata.google.internal/computeMetadata/v1/instance/attributes/sshKeys
+
+# Read keys one by one, split in case Google decided
+# to append metadata (it does sometimes) and add to
+# authorized_keys if not already present.
+touch /root/.ssh/authorized_keys
+while IFS='' read -r line || [[ -n "$line" ]]; do
+    keyLine=$(echo -n "$line" | cut -d ':' -f2)
+    IFS=' ' read -r -a array <<<"$keyLine"
+    if [[ ${#array[@]} -ge 3 ]]; then
+        echo "${array[@]:0:3}" >>/tmp/new_keys
+        echo "Added ${array[*]:2} to authorized_keys"
+    fi
+done </tmp/auth_keys
+mv /tmp/new_keys /root/.ssh/authorized_keys
+chmod 600 /root/.ssh/authorized_keys
+
+echo "Fetching host keys..."
+WGET -O /tmp/ssh_host_ed25519_key http://metadata.google.internal/computeMetadata/v1/instance/attributes/ssh_host_ed25519_key
+WGET -O /tmp/ssh_host_ed25519_key.pub http://metadata.google.internal/computeMetadata/v1/instance/attributes/ssh_host_ed25519_key_pub
+mv -f /tmp/ssh_host_ed25519_key* /etc/ssh/
+chmod 600 /etc/ssh/ssh_host_ed25519_key
+chmod 644 /etc/ssh/ssh_host_ed25519_key.pub