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

[lupyuen]

Summary

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: #14513

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

Impact

When compiling the Release Branch of nuttx-apps, our CI Workflow build.yml now checks out the Release Branch of nuttx repo.

No changes when compiling the Master Branch of nuttx and nuttx-apps repos.

Testing

Before the PR: Fetch-Source Log shows that CI is incorrectly checking out the Master Branch of nuttx repo (instead of releases/12.7): https://github.com/apache/nuttx-apps/actions/runs/11518661640/job/32069568370#step:3:66

/usr/bin/git checkout --progress --force -B master refs/remotes/origin/master

After the PR: When we change name to os_ref, CI correctly checks out the Release Branch of nuttx repo: https://github.com/lupyuen5/label-nuttx-apps/actions/runs/11528123257/job/32094821811#step:3:57

/usr/bin/git checkout --progress --force -B releases/12.7 refs/remotes/origin/releases/12.7

Regression Test:

• Creating a PR in Master Branch works correctly: https://github.com/lupyuen5/label-nuttx/actions/runs/11529724978
• Merging a PR in Master Branch also works correctly: https://github.com/lupyuen5/label-nuttx/actions/runs/11529729994

[cederom]

Thank you @lupyuen :-) Go ahead when ready :-)