Add missing input repo_url to workflow_call (#916)

Author: hjyamauchi

.github/workflows/build-toolchain.yml

@@ -62,6 +62,12 @@ on:
 
   workflow_call:
     inputs:
+      repo_url:
+        description: 'Repo Manifest URL'
+        default: 'https://github.com/compnerd/swift-build'
+        required: false
+        type: string
+
       swift_version:
         description: 'Swift Version'
         default: '0.0.0'