chore(ci): only run `deploy-code-firebase` on master

Previously, the `deploy-code-firebase` CI job was run on any build
corresponding to a git tag, the master branch or the stable branch.
Given that all runs of `deploy-code-firebase` deploy to the same
Firebase project (and overwrites previous deployments), there is no
point in deploying from multiple branches/tags.

This commit fixes this by ensuring that we only run the
`deploy-code-firebase` job on builds for the master branch, which
contains the most up-to-date code/configuration.