Fix GitHub Cargo CI by replacing unstable Arc::unwrap_or_clone

Summary:
Fixes the failure in T187825473 caused by D56339821.

This diff fixes the failing github action in https://github.com/facebook/relay/actions/runs/8932140521/job/24535481318 which was caused due to the addition of an unstable API in Arc (f7b030e). I've replaced the function with it's internal implemention from the PR to ARC at rust-lang/rust#91589.

Reviewed By: tyao1

Differential Revision: D56920263

fbshipit-source-id: e2d3f04921dd1e4b554ef075e258854b59764891

Author: evanyeung

compiler/crates/relay-compiler/src/build_project.rs

@@ -504,7 +504,8 @@ fn merge_programs(onto: &mut Programs, from: Programs) {
 }
 
 fn merge_program(onto: &mut Program, from: Arc<Program>) {
-    let from = Arc::unwrap_or_clone(from);
+    // Note: this it the inner implementation of the unstable "unwrap_or_clone"
+    let from = Arc::try_unwrap(from).unwrap_or_else(|arc| (*arc).clone());
     onto.fragments.extend(from.fragments);
     onto.operations.extend(from.operations);
 }