ci: Introduce SanityCheck task that other tasks depend on

ci: Introduce SanityCheck task that other tasks depend on

To avoid unnecessarily spinning up a lot of VMs / containers for entirely
broken commits, have a minimal task that all others depend on.

The concrete motivation for the change is to use sanitizers in the linux
tasks. As that makes the tests slower, the start of the CompilerWarnings would
be delayed even more. With this change the CompilerWarnings only depends on
the SanityCheck task.

This has the added advantage that now the CompilerWarnings task is not
prevented from running by (most) test failures (particularly annoying when
caused by a test that is flappy in HEAD).

Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/20221002205201.injtofbx4ax4erww@awork3.anarazel.de