Support `master` branch name in Actions workflow

[strugee]

Change Summary

What and Why: should be self-evident - folks who haven't/forgot to/etc. configure init.defaultBranch to main will be bit by this, i.e. the workflow will not run.

How: we do the thing. Note that I did not test this.

Related to:

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Documentation

• [ ] Fresh Produce
• [ ] In superfly/docs, or asked for help from docs team
• [x] n/a

[strugee]

Just successfully tested the GitHub Actions YAML. I still haven't tested flyctl itself. Presumably CI will be good enough because this change is so trivial.