[ci] Don't run snyk step outside of main dev branches (#101729) (#101746)

(cherry picked from commit c13350c)

Author: brianseeders

.buildkite/pipelines/periodic.template.yml

@@ -159,6 +159,7 @@ steps:
       image: family/elasticsearch-ubuntu-2004
       machineType: n2-standard-8
       buildDirectory: /dev/shm/bk
+    if: build.branch == "main" || build.branch =~ /^[0-9]+\.[0-9]+\$/
   - label: Check branch consistency
     command: .ci/scripts/run-gradle.sh branchConsistency
     timeout_in_minutes: 15

.buildkite/pipelines/periodic.yml

@@ -1350,6 +1350,7 @@ steps:
       image: family/elasticsearch-ubuntu-2004
       machineType: n2-standard-8
       buildDirectory: /dev/shm/bk
+    if: build.branch == "main" || build.branch =~ /^[0-9]+\.[0-9]+\$/
   - label: Check branch consistency
     command: .ci/scripts/run-gradle.sh branchConsistency
     timeout_in_minutes: 15