mount rp at predictable path and leave it mounted

I have a use case for wanting to manipulate the rp contents in a late-command.

examples/autoinstall/reset-only.yaml

@@ -10,4 +10,4 @@ storage:
     reset-partition: yes
     reset-partition-only: yes
 late-commands:
-  - ['true']
+  - ['sleep', '10m']

subiquity/server/controllers/install.py

@@ -389,12 +389,13 @@ async def run_curtin_step(name, stages, step_config, source=None):
         rp = fs_controller.reset_partition
         if rp is not None:
             mounter = Mounter(self.app)
-            async with mounter.mounted(rp.path) as mp:
-                await run_curtin_step(
-                    name="populate recovery", stages=["extract"],
-                    step_config=self.rp_config(logs_dir, mp.p()),
-                    source='cp:///cdrom',
-                    )
+            rp_target = os.path.join(self.app.root, 'rp')
+            mp = await mounter.mount(rp.path, mountpoint=rp_target)
+            await run_curtin_step(
+                name="populate recovery", stages=["extract"],
+                step_config=self.rp_config(logs_dir, mp.p()),
+                source='cp:///cdrom',
+                )
             await self.create_rp_boot_entry(context=context, rp=rp)
 
     @with_context(description="creating boot entry for reset partition")