[CI] Do not pull latest mutes from main when not for PR (elastic#120119) (elastic#120180)

In case when Buildkite pipeline do not run for the PR the env
`BUILDKITE_PULL_REQUEST` is set to "false".
In this case we do not want to override the `muted-tests.yml` file
with version from main.

Author: jozala

.buildkite/scripts/get-latest-test-mutes.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-if [[ ! "${BUILDKITE_PULL_REQUEST:-}" || "${BUILDKITE_AGENT_META_DATA_PROVIDER:-}" == "k8s" ]]; then
+if [[ "${BUILDKITE_PULL_REQUEST:-false}" == "false"  || "${BUILDKITE_AGENT_META_DATA_PROVIDER:-}" == "k8s" ]]; then
   exit 0
 fi