Start generated Nix expressions with a comment about automatic generation

ros2nix/ros2nix.py

@@ -17,6 +17,7 @@ import json
 import os
 import re
 import subprocess
+import sys
 
 
 def resolve_dependencies(deps: Iterable[str]) -> Set[str]:
@@ -336,7 +337,16 @@ def ros2nix(args):
             raise e
 
         try:
-            derivation_text = derivation.get_text(args.copyright_holder, args.license)
+            our_cmd_line = " ".join(
+                [os.path.basename(sys.argv[0])]
+                + [
+                    arg
+                    for arg in sys.argv[1:]
+                    if not (arg.endswith("package.xml") and os.path.isfile(arg))
+                ]
+            )
+            derivation_text = f"# Automatically generated by: {our_cmd_line}\n"
+            derivation_text += derivation.get_text(args.copyright_holder, args.license)
         except UnresolvedDependency as e:
             err(f"Failed to resolve required dependencies for package {pkg}!")
             raise e