incus: patch lxc/rootfs directory to fix tmpfs-overlay

pkgs/by-name/in/incus/generic.nix

@@ -108,6 +108,17 @@ buildGoModule (finalAttrs: {
   # required for go-cowsql.
   CGO_LDFLAGS_ALLOW = "(-Wl,-wrap,pthread_create)|(-Wl,-z,now)";
 
+  # add our lxc location to incus's acceptable rootFsPaths
+  # this is necessary for tmpfs/tmpfs-overlay to work
+  postPatch =
+    if (lib.versionAtLeast finalAttrs.version "6.16") then
+      ''
+        substituteInPlace internal/server/device/disk.go \
+          --replace-fail '"/opt/incus/lib/lxc/rootfs/"' '"${lxc}/lib/lxc/rootfs/"'
+      ''
+    else
+      null;
+
   postBuild = ''
     # build docs
     mkdir -p .sphinx/deps