feat(tvix/castore/directory/get_recursive): add 'static to BoxStream

Necessary to directly use this in the GRPC DirectoryService wrapper
directly.

Change-Id: Ic6a0038a40dc30071d145af5035345fcd93288ae
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11634
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/castore/src/directoryservice/memory.rs

@@ -73,7 +73,7 @@ impl DirectoryService for MemoryDirectoryService {
     fn get_recursive(
         &self,
         root_directory_digest: &B3Digest,
-    ) -> BoxStream<Result<proto::Directory, Error>> {
+    ) -> BoxStream<'static, Result<proto::Directory, Error>> {
         traverse_directory(self.clone(), root_directory_digest)
     }