fix lint warning after Rust update

Summary:
We are now using Rust 1.81.0. The new lint warning:

```
warning: this function depends on never type fallback being `()`
```

Forces us to use explicit types in `try_collect` when we're collecting over
fuctions returning unit types. (Previously that was a never type which was
implicitly casted to unit type).

More context: rust-lang/rust#123748

Reviewed By: diliop

Differential Revision: D64007786

fbshipit-source-id: e7da6827b799053a4a37247e80ccf9f36636d20a

Author: mitrandir77

eden/mononoke/blobstore/packblob/src/store.rs

@@ -226,7 +226,7 @@ impl<T: Blobstore + BlobstoreUnlinkOps> PackBlob<T> {
             let key = format!("{}{}{}", key_prefix, key, ENVELOPE_SUFFIX);
             links.push(self.inner.copy(ctx, &pack_key, key));
         }
-        links.try_collect().await?;
+        links.try_collect::<()>().await?;
 
         // remove the pack key, so that only the entries links are keeping it live
         self.inner.unlink(ctx, &pack_key).await?;

eden/mononoke/commit_rewriting/cross_repo_sync/src/git_submodules/validation.rs

@@ -787,7 +787,7 @@ where
             }
         })
         .buffer_unordered(100)
-        .try_collect()
+        .try_collect::<()>()
         .await?;
 
     Ok(())

eden/mononoke/git/import_tools/src/lib.rs

@@ -543,7 +543,7 @@ pub async fn import_commit_contents<Uploader: GitUploader, Reader: GitReader>(
                                 }
                             })
                             .try_buffer_unordered(100)
-                            .try_collect()
+                            .try_collect::<()>()
                             .await?;
                         // Upload packfile base item for Git commit and the raw Git commit
                         let packfile_item_upload = async {

eden/mononoke/megarepo_api/commit_transformation/src/lib.rs

@@ -829,7 +829,7 @@ pub async fn upload_commits<'a>(
             copy_file_contents(ctx, sm_repo.as_ref(), target_repo, content_ids, |_| {}).await
         })
         .buffer_unordered(10)
-        .try_collect()
+        .try_collect::<()>()
         .await?;
 
     // Then copy from source repo

eden/mononoke/server/src/main.rs

@@ -348,7 +348,7 @@ fn main(fb: FacebookInit) -> Result<()> {
                 // Repo cache warmup can be quite expensive, let's limit to 40
                 // at a time.
                 .buffer_unordered(40)
-                .try_collect()
+                .try_collect::<()>()
                 .await?;
             info!(&root_log, "Cache warmup completed");
             if let Some(mut executor) = args.sharded_executor_args.build_executor(