remove the partial salvage dir when a salvage fails

- a failure between creating the dir and the rename removes that dir only
- give the ephemeral-rootfs vm test a 4 GiB disk so the space check passes

nix/vm-tests/agent-ephemeral-rootfs.nix

@@ -78,6 +78,9 @@ pkgs.testers.runNixOSTest {
         pkgs.hello
         pkgs.tree
       ];
+      # The salvage skips when a copy would leave under 15% of the disk free, and
+      # this disk cannot clone files; the default 1 GiB leaves too little room.
+      virtualisation.diskSize = 4096;
       # The agent bridge hive-priv names in the container's conf.
       networking.bridges.${bridge}.interfaces = [ ];
       networking.interfaces.${bridge}.ipv4.addresses = [