uv cache prune removes all cached environments (#4845)

## Summary

Resolves #4802

## Test Plan
- `cargo test`
```sh
❯ cargo run -- cache prune -v
Pruning cache at: /Users/ahmedilyas/Library/Caches/uv
No unused entries found
❯ cargo run -- tool run cowsay
warning: `uv tool run` is experimental and may change without warning.
Resolved 1 package in 182ms
Installed 1 package in 20ms
 + cowsay==6.1
usage: Cowsay [-h] [-c CHARACTER] -t TEXT [-v]
Cowsay: error: the following arguments are required: -t/--text
❯ cargo run -- cache prune -v
Pruning cache at: /Users/ahmedilyas/Library/Caches/uv
    0.793440s DEBUG uv_cache Removing dangling cache entry: /Users/ahmedilyas/Library/Caches/uv/environments-v1/095cd7c4c298a0d8
Removed 41 files (143.5KiB)
```

crates/uv-cache/src/lib.rs

@@ -388,13 +388,34 @@ impl Cache {
             }
         }
 
-        for entry in fs::read_dir(self.bucket(CacheBucket::Archive))? {
-            let entry = entry?;
-            let path = entry.path().canonicalize()?;
-            if !references.contains(&path) {
-                debug!("Removing dangling cache entry: {}", path.display());
-                summary += rm_rf(path)?;
+        match fs::read_dir(self.bucket(CacheBucket::Archive)) {
+            Ok(entries) => {
+                for entry in entries {
+                    let entry = entry?;
+                    let path = entry.path().canonicalize()?;
+                    if !references.contains(&path) {
+                        debug!("Removing dangling cache entry: {}", path.display());
+                        summary += rm_rf(path)?;
+                    }
+                }
             }
+            Err(err) if err.kind() == io::ErrorKind::NotFound => (),
+            Err(err) => return Err(err),
+        }
+
+        // Third, remove any cached environments. These are never referenced by symlinks, so we can
+        // remove them directly.
+        match fs::read_dir(self.bucket(CacheBucket::Environments)) {
+            Ok(entries) => {
+                for entry in entries {
+                    let entry = entry?;
+                    let path = entry.path().canonicalize()?;
+                    debug!("Removing dangling cache entry: {}", path.display());
+                    summary += rm_rf(path)?;
+                }
+            }
+            Err(err) if err.kind() == io::ErrorKind::NotFound => (),
+            Err(err) => return Err(err),
         }
 
         Ok(summary)