debug: steam flatpak path

Signed-off-by: Shiloh Fen <[e-mail]>

src/atomic-xr/_debug.nu

···
             paths: {
                 dotsteam: (try {ls -lD ~/.steam})
                 localshare: (try {ls -lD ~/.local/share/Steam})
+                flatpak: ("~/.var/app/com.valvesoftware.Steam" | path exists)
             }
         }
         rpm-ostree: {