When running on an Alpine system, download parse-changelog from testing repository

[notgull]

I recently added parse-changelog to Alpine's testing repository here. Rather than manually downloading parse-changelog's binaries from GH releases, for Alpine we should just install it with apk.

[taiki-e]

Great news! I would accept a PR to implement this.

[taiki-e]

Currently the version of parse-changelog used in this action is tied to the version of this action, and this property is important for the stability of the action, but I wonder if doing this could break it.

Perhaps, instead, if parse-changelog is already installed we could use it, so that users who want to use the one from apk can use it.