vars: refactor - make shared generators carry machines list

This should make it simpler to improve the implementation of granting a new machine access to a shared secret.
The current approach using the health_check is  pretty hacky

pkgs/clan-cli/clan_lib/vars/generate.py

@@ -181,10 +181,10 @@ def run_generators(
     flake = machines[0].flake
 
     def get_generator_machine(generator: Generator) -> Machine:
-        if generator.machine is None:
-            # return first machine if generator is not tied to a specific one
+        if generator.share:
+            # return first machine if generator is shared
             return machines[0]
-        return Machine(name=generator.machine, flake=flake)
+        return Machine(name=generator.machines[0], flake=flake)
 
     # preheat the select cache, to reduce repeated calls during execution
     selectors = []