Fix interpreter cache collisions for relative virtualenv paths (#3823)

Closes #3784

The cache did not use an absolute path. I'm not sure this is actually a
new bug, as this code wasn't touched in #3266 but perhaps there was a
slight difference in the paths we were passing around. Note, just
canonicalizing the path as soon as we see it doesn't work because then
we jump out of the virtual environmnent into the system interpreter.

## Test plan

```
❯ uv venv
Using Python 3.12.3 interpreter at: /opt/homebrew/opt/python@3.12/bin/python3.12
Creating virtualenv at: .venv
Activate with: source .venv/bin/activate
❯ uv pip install anyio
Resolved 3 packages in 81ms
Installed 3 packages in 4ms
 + anyio==4.3.0
 + idna==3.7
 + sniffio==1.3.1
❯ mkdir uv-issue-3784 && cd uv-issue-3784
❯ uv venv
Using Python 3.12.3 interpreter at: /opt/homebrew/opt/python@3.12/bin/python3.12
Creating virtualenv at: .venv
Activate with: source .venv/bin/activate

❯ gcm
Switched to branch 'main'
Your branch is up to date with 'origin/main'.
❯ cargo run -q -- pip list -v -p .venv
DEBUG Checking for Python interpreter in directory `.venv`
TRACE Cached interpreter info for Python 3.12.3, skipping probing: .venv/bin/python3
DEBUG Using Python 3.12.3 environment at .venv/bin/python3
Package Version
------- -------
anyio   4.3.0
idna    3.7
sniffio 1.3.1
❯ cd uv-issue-3784
❯ cargo run -q -- pip list -v -p .venv
DEBUG Checking for Python interpreter in directory `.venv`
TRACE Cached interpreter info for Python 3.12.3, skipping probing: .venv/bin/python3
DEBUG Using Python 3.12.3 environment at /Users/zb/workspace/uv/.venv/bin/python3
Package Version
------- -------
anyio   4.3.0
idna    3.7
sniffio 1.3.1

❯ cd ..
❯ gco zb/fix-relative-venv
Switched to branch 'zb/fix-relative-venv'
❯ cargo run -q -- pip list -v -p .venv
DEBUG Checking for Python interpreter in directory `.venv`
TRACE Cached interpreter info for Python 3.12.3, skipping probing: .venv/bin/python3
DEBUG Using Python 3.12.3 environment at .venv/bin/python3
Package Version
------- -------
anyio   4.3.0
idna    3.7
sniffio 1.3.1
❯ cd uv-issue-3784
❯ cargo run -q -- pip list -v -p .venv
DEBUG Checking for Python interpreter in directory `.venv`
TRACE Cached interpreter info for Python 3.12.3, skipping probing: .venv/bin/python3
DEBUG Using Python 3.12.3 environment at .venv/bin/python3
```

crates/uv-interpreter/src/interpreter.rs

@@ -595,9 +595,13 @@ impl InterpreterInfo {
         let cache_entry = cache.entry(
             CacheBucket::Interpreter,
             "",
-            format!("{}.msgpack", digest(&executable)),
+            // We use the absolute path for the cache entry to avoid cache collisions for relative paths
+            // but we do not want to query the executable with symbolic links resolved
+            format!("{}.msgpack", digest(&uv_fs::absolutize_path(executable)?)),
         );
 
+        // We check the timestamp of the canonicalized executable to check if an underlying
+        // interpreter has been modified
         let modified = Timestamp::from_path(uv_fs::canonicalize_executable(executable)?)?;
 
         // Read from the cache.