CI: Check out the correct branch of nuttx repo when compiling nuttx-apps

When compiling the Release Branch of `nuttx-apps`, our CI Workflow `build.yml` checks out the Master Branch of `nuttx` repo, which is incorrect. This happens due to a typo in `build.yml`: apache/nuttx#14513

This PR fixes the typo in `build.yml` to checkout the correct branch of `nuttx` repo.

Author: lupyuen

.github/workflows/build.yml

@@ -81,7 +81,7 @@ jobs:
             esac
           fi
 
-          echo "name=$OS_REF" >> $GITHUB_OUTPUT
+          echo "os_ref=$OS_REF" >> $GITHUB_OUTPUT
           echo "apps_ref=$APPS_REF" >> $GITHUB_OUTPUT
 
       - name: Checkout nuttx repo