Add email alerts to buildkite pipelines (#1030)

Signed-off-by: Pysith Vanuptikul <piv@google.com>

Author: pv97

.buildkite/main.yml

@@ -1,6 +1,10 @@
 x-run-condition: &run-on-nightly-or-tag
   if: build.env("NIGHTLY") == "1" || build.tag =~ /^v?[0-9]+(\.[a-zA-Z0-9]+)*([a-zA-Z]+[0-9]*)?$/
 
+notify:
+  - email: "ullm-oncall@rotations.google.com"
+    if: build.state == "failed" && build.branch == "main"
+
 steps:
    - label: "Upload Tests for Models & Features"
      <<: *run-on-nightly-or-tag

.buildkite/pipeline_jax.yml

@@ -1,3 +1,7 @@
+notify:
+  - email: "ullm-oncall@rotations.google.com"
+    if: build.state == "failed" && build.branch == "main"
+
 steps:
   # -----------------------------------------------------------------
   # TEST STEPS - Calling wrapper