ElasticReindexIT: tag broken test as flaky -for CI

Add the flaky tag to acceptance/pgm/BUILD for the ElasticReindexIT test
only, so the latter can be excluded from elastic-filtered test runs.

Do so to support running the elastic-tagged tests in CI, which requires
excluding that currently failing test, so elastic tests feedback works:

bazel test //... --test_tag_filters=elastic,-flaky (e.g.).

Before this change, it was not possible to benefit from elastic tests
feedback in CI. Thus the previous risk of elastic support regressions.
So this change allows enabling the elastic-tagged tests in CI as well.

Issue 8799 is the one tracking that currently failing test. Once that
issue gets fixed, the hereby added flaky tag should be removable. A
corresponding follow-up comment is being added to that issue in turn.

Change-Id: I9bf6601bc8935caaf8dc1ccdbe4a57ee3efe78d9

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/pgm/BUILD

@@ -19,6 +19,7 @@ acceptance_tests(
     labels = [
         "docker",
         "elastic",
+        "flaky",
         "pgm",
     ],
     deps = [