[Debuginfod] Remove f-string from llvm-debuginfod-find lit test.

[noajshu]

Summary

This removes a python3 f-String from the llvm-debuginfod-find lit test script to maximize compatibility with different python versions, as it seemed to cause some issues.

[phosek]

LGTM

llvm/test/tools/llvm-debuginfod-find/debuginfod.test

httpd = http.server.ThreadingHTTPServer(
         ('',0), functools.partial(
             http.server.SimpleHTTPRequestHandler,
             directory=server_path))
     port = httpd.server_port
     thread = threading.Thread(target=httpd.serve_forever)
     try:
         thread.start()
         env = os.environ
-        env['DEBUGINFOD_URLS'] = f'http://localhost:%s' % port
+        env['DEBUGINFOD_URLS'] = 'http://localhost:%s' % port
         process = subprocess.Popen(
             tool_args, env=env)
         code = process.wait()
         if code != 0:
             print('nontrivial return code %s' % code)
             return 1
     finally:
         httpd.shutdown()