ci: migration: Don't run python tests in the compat job

ci: migration: Don't run python tests in the compat job

The vmstate-checker-script test has a bug that makes it flaky. It was
also committed by mistake and will be removed.

Since the migration-compat job takes the tests from the build-previous
job instead of the current HEAD, neither a fix or a removal of the
test will take effect for this release.

Disable the faulty/undesirable test by taking advantage that it only
runs if the PYTHON environment variable is set. This also disables the
analyze-migration-script test, but this is fine because that test
doesn't have migration compatibility implications.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Link: https://lore.kernel.org/r/20240905185445.8179-1-farosas@suse.de
[peterx: Added a TODO to remove the line after 9.2 release, per thuth]
Signed-off-by: Peter Xu <peterx@redhat.com>