Clean up release-to-master action

[jsalankey]

The release-to-master Github Action was not working since forever due to a missing required environment variable. Upon fixing that, I encountered another issue with a duplicate branch causing the action to fail to run. According to robotology/gh-action-nightly-merge#4, this issue was fixed in 1.3.2, but it's still failing the merge action.

This can be fixed by the change in 668f4c5 and using 1.3.1 of the checkout action, but this feels hacky. Try to figure out why the action is failing to merge when using checkout@v2 and merge@1.3.2.