pacman can't find package file added to custom repo database

Question

I've added a package libxau-1.0.11-1-x86_64.pkg.tar.zst to my custom repository database custom.db.tar.gz using repo-add. The package file is located in the current directory, while the repo database is in the parent directory:

repo-add ../custom.db.tar.gz ./libxau-1.0.11-1-x86_64.pkg.tar.zst

This runs without any errors. However, when I try to install the package with:

pacman -S libxau

I get the following error:

libxau-1.0.11-1-x86_64.pkg.tar.zst failed to download
error: failed retrieving file 'libxau-1.0.11-1-x86_64.pkg.tar.zst' from disk : Couldn't open file /self/libxau-1.0.11-1-x86_64.pkg.tar.zst

It seems pacman is expecting the package file to be in the same directory as the repo database file, which is not the case here. the file would be at /self/libxau/libxau-1.0.11-1-x86_64.pkg.tar.zst

How can I get pacman to look in the correct directory for the package file that was added to the custom repo database? Is there a way to specify the package file path relative to the repo database? Or do I need to move the package file to the same directory as the database?

Any suggestions on how to resolve this would be appreciated. Let me know if you need any other details.