build-support/vm: fix unpopulated debsGrouped after structuredAttrs refactor

debsGrouped was a nested array, like `[ [ drv1 drv2 ] [ drv3 ] ... ]`.
that doesn't map to any bash item, so it would live only in the json attrs.

the `for component in "${debsGrouped[@]}"; ...` part of buildCommand
was thus referring to an unset variable... but the default bash behavior
is to treat that as an empty array even with `set -u` so the loop was
just doing nothing.

instead, flatten the array on the nix side. bash gets something like
`("drv1 drv2" "drv3" ...)` and the existing shell code destructures
that as expected.

verify by building a vm (e.g.
`tests.vmTools.buildPatchelfInDebian.diskImage`) and checking the
nix-log before and after for `INSTALLING COMPONENT` messages.

Co-Authored-By: Stefan Frijters <sfrijters@gmail.com>

pkgs/build-support/vm/default.nix

@@ -731,7 +731,7 @@ let
             ;
 
           debsFlat = lib.flatten debs;
-          debsGrouped = debs;
+          debsGrouped = map toString debs;
 
           preVM = createEmptyImage { inherit size fullName; };
 

pkgs/build-support/vm/test.nix

@@ -2,12 +2,12 @@
   hello,
   patchelf,
   pcmanfm,
+  runCommand,
   stdenv,
   vmTools,
 }:
 let
   inherit (vmTools)
-    buildRPM
     diskImages
     makeImageTestScript
     runInLinuxImage
@@ -46,6 +46,23 @@ in
     })
   );
 
+  # Sanity check to ensure the dpkg --install commands have run
+  checkPerlInstalledInDebian = runInLinuxImage (
+    runCommand "check-perl"
+      {
+        diskImage = diskImages.debian13x86_64;
+        diskImageFormat = "qcow2";
+        memSize = 512;
+      }
+      ''
+        echo Check if perl is present
+        perl -v
+        echo Check if perl is installed
+        dpkg -l | grep 'ii *perl'
+        mkdir $out
+      ''
+  );
+
   # RPM-based distros
   testFedora42Image = makeImageTestScript diskImages.fedora42x86_64;
   testFedora43Image = makeImageTestScript diskImages.fedora43x86_64;