Add workflow to update Nimbus first-run experiments

[jeddai]

Problem

The first-run experiments are updated asynchronously to the firefox-ios build process, and should be kept up to date more frequently.

Proposed Solution

Create a github action workflow that will periodically update the first-run experiments from the experimenter API. It should write the changes to the repository and open a new pull request for them.

Epic - Issue (EXP-2628)

┆Issue is synchronized with this Jira Task