fix: manually specify venv when installing invokeai pkg

[psychedelicious]

This fixes an issue @shanedk had, where uv failed to find the virtual environment for the Invoke install.

I had made a custom builder for them, but not yet merged this fix into main.

[shanedk]

Sorry, but it looks like this hasn't been fixed after all.

UPDATE: I was able to get it working with Repair mode.

Installation parameters:
- Install location: W:\AI\InvokeAI
- GPU type: nvidia>=30xx
- Torch Platform: cuda
Virtual environment already exists, skipping...
Installing invokeai package...
> C:\Users\shane\AppData\Local\Temp\2vFxKvvFbjMAsUlvZ94N07dukNe\resources\bin\uv.exe pip install --python 3.12 --python-preference only-managed invokeai==v5.10.0a1 --force-reinstall --compile-bytecode --index=https://download.pytorch.org/whl/cu126
error: Failed to inspect Python interpreter from virtual environment at `Scripts\python.exe`
  Caused by: Python interpreter not found at `W:\AI\InvokeAI\Scripts\python.exe`
Failed to install invokeai python package: Process exited with code 2
Try installing again with Repair mode enabled to fix this.
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