Set env BAZELCI_TASK (#1654)

so that subprocesses are able to know which task/step BazelCI is
currently running.

Working towards Test Analytics integration.
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 7febd1a..3151196 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -3909,6 +3909,8 @@
                     )
                 )
 
+            os.environ["BAZELCI_TASK"] = args.task
+
             platform = get_platform_for_task(args.task, task_config)
 
             # The value of `BUILDKITE_MESSAGE` defaults to the commit message, which can be too large