USN-5292-4: snapd regression

USN-5292-1 fixed a vulnerability in snapd. Unfortunately that update introduced
a regression that could break the fish shell. This update fixes the problem.

We apologize for the inconvenience.

Original advisory details:

James Troup discovered that snap did not properly manage the permissions for
the snap directories. A local attacker could possibly use this issue to expose
sensitive information. (CVE-2021-3155)

Ian Johnson discovered that snapd did not properly validate content interfaces
and layout paths. A local attacker could possibly use this issue to inject
arbitrary AppArmor policy rules, resulting in a bypass of intended access
restrictions. (CVE-2021-4120)

The Qualys Research Team discovered that snapd did not properly validate the
location of the snap-confine binary. A local attacker could possibly use this
issue to execute other arbitrary binaries and escalate privileges.
(CVE-2021-44730)

The Qualys Research Team discovered that a race condition existed in the snapd
snap-confine binary when preparing a private mount namespace for a snap. A
local attacker could possibly use this issue to escalate privileges and
execute arbitrary code. (CVE-2021-44731)