Flatpak-builder: local file exfiltration via `file

A flaw was found in flatpak-builder. This vulnerability allows an attacker to cause information disclosure by convincing a user or continuous integration (CI) system to process a crafted build manifest. By specifying local file Uniform Resource Identifiers (URIs) within source download definitions, the builder bypasses directory confinement checks. As a result, sensitive host files accessible to the build process can be read and incorporated into the build artifacts.