Cache tool environments in uv tool run (#4784)

## Summary

The basic strategy:

- When the user does `uv tool run`, we resolve the `from` and `with`
requirements (always).
- After resolving, we generate a hash of the requirements. For now, I'm
just converting to a lockfile and hashing _that_, but that's an
implementation detail.
- Once we have a hash, we _also_ hash the interpreter.
- We then store environments in
`${CACHE_DIR}/${INTERPRETER_HASH}/${RESOLUTION_HASH}`.

Some consequences:

- We cache based on the interpreter, so if you request a different
Python, we'll create a new environment (even if they're compatible).
This has the nice side-effect of ensuring that we don't use environments
for interpreters that were later deleted.
- We cache the `from` and `with` together. In practice, we may want to
cache them separately, then layer them? But this is also an
implementation detail that we could change later.
- Because we use the lockfile as the cache key, we will invalidate the
cache when the format changes. That seems ok, but we could improve it in
the future by generating a stable hash from a lockfile that's
independent of the schema.

Closes https://github.com/astral-sh/uv/issues/4752.

crates/cache-key/src/lib.rs

@@ -1,3 +1,4 @@
+pub use cache_key::{CacheKey, CacheKeyHasher};
 pub use canonical_url::{CanonicalUrl, RepositoryUrl};
 pub use digest::digest;