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/modules/virtualisation/qemu-vm.nix

@@ -186,7 +186,7 @@ let
         NIX_EFI_VARS=$(readlink -f "''${NIX_EFI_VARS:-${config.system.name}-efi-vars.fd}")
         # VM needs writable EFI vars
         if ! test -e "$NIX_EFI_VARS"; then
-        ${if cfg.useBootLoader then
+        ${if cfg.useBootLoader && cfg.useDefaultFilesystems then
             # We still need the EFI var from the make-disk-image derivation
             # because our "switch-to-configuration" process might
             # write into it and we want to keep this data.