hopefully IFD was the problem

pkgs/applications/networking/misc/zammad/default.nix

@@ -52,7 +52,7 @@ let
     #   https://docs.zammad.org/en/latest/prerequisites/software.html#ruby-programming-language
     inherit ruby_2_7;
 
-    gemdir = "${src}";
+    gemdir = src;
     gemset = ./gemset.nix;
     groups = [
       "assets"
@@ -89,7 +89,7 @@ let
     inherit version src;
     yarnLock = ./yarn.lock;
     yarnNix = ./yarn.nix;
-    packageJSON = "${src}/package.json";
+    packageJSON = ./package.json;
   };
 
 in

pkgs/applications/networking/misc/zammad/package.json

@@ -0,0 +1,14 @@
+{
+  "name": "Zammad",
+  "version": "1.0.0",
+  "devDependencies": {
+    "gulp": "^3.8.11",
+    "gulp-cheerio": "^0.6.2",
+    "gulp-rename": "^1.2.2",
+    "gulp-svgmin": "^1.1.2",
+    "gulp-svgstore": "^5.0.1",
+    "gulp-util": "^3.0.4",
+    "gulp-watch": "^4.2.4",
+    "through2": "^0.6.5"
+  }
+}

pkgs/applications/networking/misc/zammad/update.sh

@@ -60,6 +60,9 @@ yarn install
 cp yarn.lock $TARGET_DIR
 yarn2nix > $TARGET_DIR/yarn.nix
 
+# needed to avoid import from derivation
+cp package.json $TARGET_DIR
+
 popd
 popd
 popd