Update Nightly Build Process to use Feature Branch or Master

[dstadulis]

Current, the nightly build process is set up to build from an experimental branch, which will eventually be merged into master. To ensure the CD process remains up-to-date, the on-going branch used to build nightlys should be set to either a pre-release branch (say feature/new-feature) or based on master.

To resolve this issue, the nightly build process should be updated to use one of the following approaches:

1. Feature Branch: Configure the nightly build repo to build from a specific feature branch (e.g. feature/new-feature) that is regularly updated with the latest changes.
2. Master Branch: Update the nightly build repo to build directly from the master branch, which will ensure that our builds always reflect the latest codebase.

TODO: Once this issue is resolved, the # TODO(guggero): Change the branch to 'master' once the experimental branch is merged into master. comment can be removed.

https://github.com/lightninglabs/lightning-terminal/blob/2c6075328f89094ba03437ed6cae197ad7d5365b/.github/workflows/docker.yml#L21-L22