nixos/tests/installer: avoid create_machine, clean up

- use normal VM nodes for target, with some extra trickery
- rename preBootCommands to postBootCommands to match its actual intent
- rename VMs to installer and target, so they're not all called machine
- set platforms on non-UEFI tests properly
- add missing packages for systemd-boot test
- fix initrd secrets leaking into the store and having wrong paths

nixos/lib/testing/meta.nix

@@ -34,6 +34,13 @@ in
               Sets the [`meta.broken`](https://nixos.org/manual/nixpkgs/stable/#var-meta-broken) attribute on the [{option}`test`](#test-opt-test) derivation.
             '';
           };
+          platforms = lib.mkOption {
+            type = types.listOf types.raw;
+            default = lib.platforms.linux;
+            description = ''
+              Sets the [`meta.platforms`](https://nixos.org/manual/nixpkgs/stable/#var-meta-platforms) attribute on the [{option}`test`](#test-opt-test) derivation.
+            '';
+          };
         };
       };
       default = {};