Randomly assign PORT for PGO.

It appears a process on GHA Linux now uses 8888.

build/pgo/profileserver.py

@@ -7,6 +7,7 @@
 import glob
 import json
 import os
+import socket
 import subprocess
 import sys
 
@@ -19,7 +20,12 @@
 from mozrunner import CLI, FirefoxRunner
 from six import string_types
 
-PORT = 8888
+def find_free_port():
+    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
+        s.bind(('', 0))
+        return s.getsockname()[1]
+
+PORT = find_free_port()
 
 PATH_MAPPINGS = {
     "/webkit/PerformanceTests": "third_party/webkit/PerformanceTests",