Fix plugin cache panic when cwd is unavailable (#18499)

## Summary

Fixes #16637. (I hit this bug after 11h of work on a long-running task.)

Plugin cache initialization could panic when an already-absolute cache
path was normalized through `AbsolutePathBuf::from_absolute_path`,
because that path still consulted `current_dir()`.

This changes absolute-path normalization so already-absolute paths do
not depend on cwd, and makes plugin cache root construction available as
a fallible path through `PluginStore::try_new()`. Plugin cache subpaths
now use `AbsolutePathBuf::join()` instead of re-absolutizing derived
absolute paths.

codex-rs/utils/absolute-path/src/absolutize.rs

@@ -12,6 +12,10 @@ use std::path::Path;
 use std::path::PathBuf;
 
 pub(super) fn absolutize(path: &Path) -> std::io::Result<PathBuf> {
+    if path.is_absolute() {
+        return Ok(normalize_path(path));
+    }
+
     Ok(absolutize_from(path, &std::env::current_dir()?))
 }