DiffusionGemma: set UNSLOTH_IS_PRESENT for the shim subprocess (#6259)

A clean install could not run DiffusionGemma: the runner spawns
python -m unsloth_zoo.diffusion_studio.shim, and unsloth_zoo refuses to
import unless UNSLOTH_IS_PRESENT is set (normally by import unsloth). The
shim never imports unsloth, so the subprocess died with
'Please install Unsloth via pip install unsloth!' and the model load
failed with a 500. Set the flag in the runner child env, as unsloth does
on import.

studio/backend/core/inference/llama_cpp.py

@@ -2527,6 +2527,10 @@ class LlamaCppBackend:
         ]
 
         env = child_env_without_native_path_secret()
+        # `python -m unsloth_zoo.diffusion_studio.shim` imports unsloth_zoo, which
+        # refuses to load unless UNSLOTH_IS_PRESENT is set (normally by `import
+        # unsloth`). The shim never imports unsloth, so set it here as unsloth does.
+        env["UNSLOTH_IS_PRESENT"] = "1"
         env["DG_VISUAL_BIN"] = visual_bin
         env["DG_GPU"] = gpu
         # The file-override shim imports its sibling visual_engine; put its dir on PYTHONPATH.