fix: writable_roots doesn't recognize home directory symbol in non-windows OS (#9193)

Fixes:
```
[sandbox_workspace_write]
writable_roots = ["~/code/"]
```

translates to
```
/Users/ccunningham/.codex/~/code
```
(i.e. the home dir symbol isn't recognized)

codex-rs/Cargo.lock

@@ -1888,7 +1888,9 @@ dependencies = [
 name = "codex-utils-absolute-path"
 version = "0.0.0"
 dependencies = [
+ "dirs",
  "path-absolutize",
+ "pretty_assertions",
  "schemars 0.8.22",
  "serde",
  "serde_json",