ci: don't run minimum bazel version tests as part of bazel downstream… (#1522)

The Bazel downstream tests will use Bazel built at head, but the tests
checking for support with the minimum Bazel version are specifically
intended to only run with an older Bazel version.

Work towards #1520

.bazelci/presubmit.yml

diff --git a/.bazelci/presubmit.yml b/.bazelci/presubmit.yml
index 491c685..f21c423 100644
--- a/.bazelci/presubmit.yml
+++ b/.bazelci/presubmit.yml
@@ -22,6 +22,7 @@
   # For testing minimum supported version.
   # NOTE: Keep in sync with //:version.bzl
   bazel: 5.4.0
+  skip_in_bazel_downstream_pipeline: "Bazel 5 required"
 .minimum_supported_bzlmod_version: &minimum_supported_bzlmod_version
   bazel: 6.2.0 # test minimum supported version of bazel for bzlmod tests
 .reusable_config: &reusable_config