Can't find source path /mnt: Permission denied

`bbwrap` fails to run when an sshfs is already mounted under /mnt as root on the host.

versions:
------------
bubblewrap-0.4.1-2.fc33.x86_64
flatpak-1.10.2-3.fc33.x86_64

reproduce
-------------
1. mount an SSHFS from anywhere **as root**, under `/mnt`
2. run as **regular user** any flatpak that you may have installed (it fails with all of them)

```
$ /usr/bin/flatpak run -v --branch=stable --arch=x86_64 --command=teams --file-forwarding com.microsoft.Teams
F: No installations directory in /etc/flatpak/installations.d. Skipping
F: Opening system flatpak installation at path /var/lib/flatpak
F: Opening user flatpak installation at path /home/fabrice/.local/share/flatpak
F: Opening user flatpak installation at path /home/fabrice/.local/share/flatpak
F: Opening system flatpak installation at path /var/lib/flatpak
F: Opening user flatpak installation at path /home/fabrice/.local/share/flatpak
F: Opening system flatpak installation at path /var/lib/flatpak
F: Cleaning up unused container id 756181348
F: Allocated instance id 1018323690
F: Add defaults in dir /com/microsoft/Teams/
F: Add locks in dir /com/microsoft/Teams/
F: Allowing x11 access
F: Allowing pulseaudio access
F: Pulseaudio user configuration file '/home/fabrice/.config/pulse/client.conf': Error opening file /home/fabrice/.config/pulse/client.conf: No such file or directory
F: Running 'bwrap --args 41 xdg-dbus-proxy --args=43'
bwrap: Can't find source path /mnt: Permission denied
error: Failed to sync with dbus proxy
```
I'm not sure what `bbwrap` is trying to do with `/mnt` here.