[thread-cert] cache Border Router test results (#7303)

This commit caches Border Router test results so that success BR tests
will be skipped when re-running the workflow.

The cache keys are chosen in a way that:
- Each new PR push will trigger cache miss and run the tests
- Each ot-br-posix change will trigger cache miss and run the tests

tests/scripts/thread-cert/thread_cert.py

@@ -32,6 +32,7 @@ import json
 import logging
 import os
 import signal
+import stat
 import subprocess
 import sys
 import time
@@ -522,6 +523,7 @@ class TestCase(NcpSupportMixin, unittest.TestCase):
         time.sleep(0.2)
         assert self._dumpcap_proc.poll() is None, 'tshark terminated unexpectedly'
         logging.info('Backbone sniffer launched successfully: pid=%s', self._dumpcap_proc.pid)
+        os.chmod(pcap_file, stat.S_IWUSR | stat.S_IRUSR | stat.S_IRGRP | stat.S_IROTH)
 
     def _get_backbone_pcap_filename(self):
         backbone_pcap = self.test_name + '_backbone.pcap'