idempotent installation

Don't run the installer if Nix has already been installed on the host.
We assume that the installed Nix is already in the desired state.

This allows to share a given pipeline between hosted and self-hosted
runners. Usually the self-hosted runners are long-lived and already have
Nix installed.

lib/install-nix.sh

@@ -1,6 +1,11 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
+if type -p nix &>/dev/null ; then
+  echo "Aborting: Nix is already installed at $(type -p nix)"
+  exit
+fi
+
 # Configure Nix
 add_config() {
   echo "$1" | sudo tee -a /tmp/nix.conf >/dev/null