cloud-hypervisor: clean up the short-circuiting logic

pkgs/cloud-hypervisor-gpu.nix

@@ -34,10 +34,10 @@ cloud-hypervisor.overrideAttrs (
       patches = patchesFromDir (spectrum + "/pkgs/cloud-hypervisor");
       vhostPatches = patchesFromDir (spectrum + "/pkgs/cloud-hypervisor/vhost");
     };
-    previouslyPatched = oldAttrs ? spectrumPatches;
-    patchPhases = !previouslyPatched;
+    oldPatchesStruct = oldAttrs.passthru.spectrumPatches or { };
+    missingPatchPhases = previouslyPatched != { };
     isNewer = lib.versionOlder oldAttrs.spectrumPatches.version or "2000-00-00" spectrumPatches.version;
-    oldPatches = oldAttrs.spectrumPatches.patches or [ ];
+    oldPatches = oldPatchesStruct.patches or [ ];
     removeAll = removeElts: lst: builtins.filter (x: !(builtins.elem x removeElts)) lst;
   in
   optionalAttrs isNewer {
@@ -47,7 +47,7 @@ cloud-hypervisor.overrideAttrs (
     # Verbatim from spectrum
     postUnpack =
       oldAttrs.postUnpack or ""
-      + optionalString patchPhases ''
+      + optionalString missingPatchPhases ''
         unpackFile $vhost
         chmod -R +w vhost
       '';
@@ -58,7 +58,7 @@ cloud-hypervisor.overrideAttrs (
     # Verbatim copy from spectrum
     postPatch =
       oldAttrs.postPatch or ""
-      + optionalString patchPhases ''
+      + optionalString missingPatchPhases ''
         pushd ../vhost
         for patch in $vhostPatches; do
             echo applying patch $patch