Persist files added to /etc past system updates in SteamOS 3.6+ (#1355)

* Add atomic update persistence list for SteamOS 3.6+

* Apply suggestions from code review and fix pushing the actual list

Co-authored-by: Cole Helbling <cole.helbling@determinate.systems>

* fixup: create atomic-update.conf.d entry only if directory exists

* fixup: typo in variable name

steamos, not streamos lol

* fixup: path in test fixture

---------

Co-authored-by: Cole Helbling <cole.helbling@determinate.systems>

Author: 5310

src/planner/steam_deck.rs

@@ -254,10 +254,10 @@ impl Planner for SteamDeck {
             .map_err(PlannerError::Action)?;
             actions.push(create_bind_mount_unit.boxed());
         } else {
-            let revert_clean_streamos_nix_offload = RevertCleanSteamosNixOffload::plan()
+            let revert_clean_steamos_nix_offload = RevertCleanSteamosNixOffload::plan()
                 .await
                 .map_err(PlannerError::Action)?;
-            actions.push(revert_clean_streamos_nix_offload.boxed());
+            actions.push(revert_clean_steamos_nix_offload.boxed());
 
             let ensure_steamos_nix_directory = EnsureSteamosNixDirectory::plan()
                 .await
@@ -270,6 +270,27 @@ impl Planner for SteamDeck {
             actions.push(start_nix_mount.boxed());
         }
 
+        if std::path::Path::new("/etc/atomic-update.conf.d").exists() {
+            let create_atomic_update_buf = "\
+                /etc/fish/conf.d/nix.fish\n\
+                /etc/nix/**\n\
+                /etc/profile.d/nix.sh\n\
+                /etc/systemd/system/nix-daemon.socket\n\
+                /etc/tmpfiles.d/nix-daemon.conf\n\
+            ";
+            let create_atomic_update_unit = CreateFile::plan(
+                "/etc/atomic-update.conf.d/nix-installer.conf",
+                None,
+                None,
+                0o0644,
+                create_atomic_update_buf.to_string(),
+                false,
+            )
+            .await
+            .map_err(PlannerError::Action)?;
+            actions.push(create_atomic_update_unit.boxed());
+        }
+
         let ensure_symlinked_units_resolve_buf = "\
             [Unit]\n\
             Description=Ensure Nix related units which are symlinked resolve\n\

tests/fixtures/linux/steam-deck.json

@@ -55,6 +55,18 @@
       },
       "state": "Completed"
     },
+    {
+      "action": {
+        "action_name": "create_file",
+        "path": "/etc/atomic-update.conf.d/nix-installer.conf",
+        "user": null,
+        "group": null,
+        "mode": 420,
+        "buf": "/etc/fish/conf.d/nix.fish\n/etc/nix/**\n/etc/profile.d/nix.sh\n/etc/systemd/system/nix-daemon.socket\n/etc/tmpfiles.d/nix-daemon.conf\n",
+        "force": false
+      },
+      "state": "Completed"
+    },
     {
       "action": {
         "action_name": "start_systemd_unit",