resolve TempDir paths before passing them to the flake bin

On MacOS /tmp is a symlink to /private/tmp.
TempDir creates a dir and returns the path as /tmp.
nix is unhappy with symlink dirs when calling nix flake metadata --json

To fix this we canonicalize the path first, resolving any symlinks on the way.

src/nix/hive/assets.rs

@@ -51,7 +51,10 @@ impl Assets {
             // We explicitly specify `path:` instead of letting Nix resolve
             // automatically, which would involve checking parent directories
             // for a git repository.
-            let uri = format!("path:{}", temp_dir.path().to_str().unwrap());
+            let uri = format!(
+                "path:{}",
+                temp_dir.path().canonicalize().unwrap().to_str().unwrap()
+            );
             let _ = lock_flake_quiet(&uri).await;
             let assets_flake = Flake::from_uri(uri).await?;
             assets_flake_uri = Some(assets_flake.locked_uri().to_owned());