Fix workflow that automatically publishes @firebase/app

[hsubox76]

@firebase/app should be published on every release. The script to automatically do this was failing because yarn changesets status couldn't find the master branch locally, because actions/checkout doesn't check it out locally. After reproducing locally, it does seem to be able to access origin/master, so changed it to that.

[changeset-bot]

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Latest commit: 7b5fec0

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[Feiyang1]

LG. Thanks. Can we run the workflow in Github to make sure it actually works?

[hsubox76]

I don't think we can test the workflow without merging it because reproducing the exact git state means it has to be on a push to the release branch, and that won't trigger if it's not merged to master. I reproduced it as much as possible locally by manually doing the same weird git checkout steps that actions/checkout does.