osbuild: make state handling generic

Rather than treating the dnf-cache specially, give each stage its
own state directory that they can reuse. This should obviously be
used with care by the stages in order to make the builds
reproducible.

osbuild

@@ -47,9 +47,14 @@ def main(pipeline_path, from_archive, save):
             name = stage["name"]
             options = stage.get("options", {})
             options["tree"] = os.path.abspath(tree)
+            options["state"] = f"{os.getcwd()}/state/{name}"
 
             options_str = json.dumps(options, indent=2)
 
+            r = subprocess.run(["mkdir", "-p", f"{os.getcwd()}/state/{name}"])
+            if r.returncode != 0:
+                return
+
             print()
             print(f"{RESET}{BOLD}{i}. {name}{RESET} {options_str}")
             print()