Codesign: Wait for unsigned directory to appear

Solves problem with different order of codesign server startup and
mount of network shares: avoids exception happening when server is
started prior to the mounts are ready.

build_files/buildbot/codesign/base_code_signer.py

@@ -337,6 +337,11 @@ class BaseCodeSigner(metaclass=abc.ABCMeta):
 
         # TOOD(sergey): Support graceful shutdown on Ctrl-C.
 
+        logger_server.info(
+            f'Waiting for a request directory {self.unsigned_storage_dir} to appear.')
+        while not self.unsigned_storage_dir.exists():
+            time.sleep(1)
+
         logger_server.info(
             'Waiting for a READY indicator of any signing request.')
         request_id = None