[artifacts] Only upload beats artifacts if they exist (elastic#132136)

* [artifacts] Only upload beats artifacts if they exist

Beats artifacts are downloaded as a side effect  of building the cloud
image.  At times (e.g. a staging build, where we don't have the correct
sha) we may skip the cloud image build and these artifacts will not be
available to upload.

* Update .buildkite/scripts/steps/artifacts/build.sh

Co-authored-by: Spencer <email@spalger.com>

Co-authored-by: Spencer <email@spalger.com>

.buildkite/scripts/steps/artifacts/build.sh

@@ -25,7 +25,9 @@ buildkite-agent artifact upload "dependencies-$FULL_VERSION.csv.sha512.txt"
 buildkite-agent artifact upload 'i18n/*.json'
 cd -
 
-cd .beats
-buildkite-agent artifact upload 'metricbeat-*'
-buildkite-agent artifact upload 'filebeat-*'
-cd -
+if [ -d .beats ]; then
+  cd .beats
+  buildkite-agent artifact upload 'metricbeat-*'
+  buildkite-agent artifact upload 'filebeat-*'
+  cd -
+fi