private mount doesn't work when launching applications from within /tmp

[Jamie Strandboge]

Thanks to Christian Ehrhardt (paelzer) for finding it, and John Lenton (chipaca) for boiling it down into a digestible mulch.

$ cd /tmp
$ mkdir foo
$ hello-world.sh # from snap install hello-world
bash-4.3$ touch foo/bar
bash-4.3$ exit
$ ls /tmp/foo/
bar

We consider this a bug and not a security vulnerability because it is not attacker controlled.