bubblewrap refuses to work with an unreachable automount

If you setup a systemd automount similar to this:

```
# var-home-user-mount.automount
[Unit]
Description=auto mount
StartLimitIntervalSec=0
Requires=network.target

[Automount]
Where=/var/home/user/mount
TimeoutIdleSec=10min

[Install]
WantedBy=remote-fs.target
```

```
# var-home-user-mount.mount
[Unit]
Description=mount
Requires=network.target
StartLimitIntervalSec=0

[Mount]
What=//10.0.0.1/c
Where=/var/home/user/mount
Options=uid=1000,gid=1000,nofail,mfsymlinks
Type=cifs
```

~~And place bwrap/flatpak application desktop file into xdg-autostart directory (~/.config/autostart),~~ (← this is unnecessary. just an unreachable automount is enough to reproduce the issue) it'll stuck for a very long time then give up:

```
bwrap: Can't bind mount /oldroot/var on /newroot/var: Unable to remount recursively with correct flags: No such device
```

For instance, ALL my flatpak apps refuse to autostart just because an automount exists, the same error spams in `journalctl`. Disabling the automount fixes the issue.

```bash
> bwrap --version
bubblewrap 0.5.0
```