reuse env if present #127
Description
if I'm running a script once or twice while iterating on a function, it's nbd for uv run to rebuild the environment every time. but the resolution / venv creation process won't scale well at all (especially on HPC filesystems) when I'm running the same function 1000s of times.
a good first pass is probably just to create a venv explicitly under the hash of the script contents so we can just point uv at the known valid environment every time.