release-21.2: acceptance: update how we build the python compose image

[rickystewart]

Backport from #81313.

Up until this point we were using apt install postgresql-client to
install the psql binary in this image. Because the postgresql-client
apt package was apparently updated to a later version of postgres
last week that includes this change,
the TestComposeGSSPython acceptance test started failing with the
following error:

psql: error: private key file "/certs/client.root.key" must be owned by the current user or root

Less recent versions of psql are more permissive about cert
permissions, so we work around this by manually installing postgres 11.

Release note: None
Release justification: Test-only change

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is