Do not clear externally provided workdir in UpdateCorpusDatabase.

This is a NFC to FuzzTest since centipede_adaptor guarantees that any provided workdir is empty.

It allows syncing a corpus database with an existing workdir, enabling e.g. importing corpus from external source or another corpus database.

PiperOrigin-RevId: 983883563

centipede/centipede_interface.cc

diff --git a/centipede/centipede_interface.cc b/centipede/centipede_interface.cc
index e0793d1..44ba0c9 100644
--- a/centipede/centipede_interface.cc
+++ b/centipede/centipede_interface.cc
@@ -516,7 +516,7 @@
     }
   }
 
-  if (RemotePathExists(env.workdir) && !is_resuming) {
+  if (!is_workdir_specified && RemotePathExists(env.workdir) && !is_resuming) {
     // This could be a workdir from a failed run that used a different version
     // of the binary. We delete it so that we don't have to deal with
     // the assumptions under which it is safe to reuse an old workdir.