Specify CMS branch using env config

[dhruvparekh12]

Many a times, users try to deploy the contentstack-nextjs-starter-app on Launch using a delivery token scoped to a Contentstack branch, other than the default main branch. But, the deployments fail with this error

Error: {"error_message":"Access denied. You have insufficient permissions to perform operation on this branch 'main'.","error_code":901}

To fix this, users need to be able to optionally set an environment variable CONTENTSTACK_BRANCH mentioning the name of the branch on Contentstack.
Code changes in this PR will allow users to set CONTENTSTACK_BRANCH environment variable so their deployments can be successful on Launch.

[dhruvparekh12]

Hi @Amitkanswal @surajcontentstack @abhishek305
Can you please review and approve these changes?

[abhishek305]

Hi @dhruvparekh12,
Could you please add a description/changes made giving context?

Note: Changed the base branch to staging

cc @Amitkanswal @surajcontentstack

[dhruvparekh12]

Hi @abhishek305

Added description. Also please let me know if any additional information is required.

[dhruvparekh12]

Hi team,
Can we merge this?
I do not have permission to merge, can you please complete the merge on this PR?
Thanks

[systemsconsciousness]

Testing this change, and it is working. -DH