[8.11](backport #3742) [CI] run MacOS 13 ARM unit tests only on main …

…branch (#3745)

* [CI] run MacOS 13 ARM unit tests only on main branch (#3742)

(cherry picked from commit 3155ea9)

# Conflicts:
#	.buildkite/pipeline.yml

* Update pipeline.yml

* Update pipeline.yml

---------

Co-authored-by: Pavel Zorin <pavel.zorin@elastic.co>
Co-authored-by: Pierre HILBERT <pierre.hilbert@elastic.co>

.buildkite/pipeline.yml

@@ -80,9 +80,11 @@ steps:
       manual:
         allowed: true
 
+  # Runs only on the main branch
   - label: "Unit tests - MacOS 13 ARM"
     key: "unit-tests-macos-13-arm"
     command: ".buildkite/scripts/steps/unit-tests.sh"
+    branches: main
     artifact_paths:
       - "build/TEST-**"
       - "build/diagnostics/*"
@@ -119,7 +121,6 @@ steps:
       unit-tests-win2016
       unit-tests-win2022
       unit-tests-macos-13
-      unit-tests-macos-13-arm
       "
     artifact_paths:
       - "build/TEST-**"