Only build stable on version change and in virtualmin repo

[swelljoe]

Hopefully, this will make the following changes:

1. Only build stable packages if the module.info has changed (hopefully indicating someone remembered to bump the version).
2. Removes date string from the stable package version (since we will hopefully always have a new human selected version).
3. Only build packages if it's on the virtualmin repo, not forks. Tests still run on forks, so people can see if they've made a silly mistake, like syntax errors. Forks shouldn't have valid secrets for our repos, so even if it builds on a fork it can't actually publish, but no reason to chew CPU cycles for nothing.
4. Fixes some problems with publishing (permissions and ownership), so the build server user can operate on files created by the actions user.
5. Changes the publish queue path to be the full file path. The publisher splits it at run time so it can both sign the package with the right key for each repo, and publish any repos with changes.

Hopefully this is the last of the bits needed before I start replicating this out to all Virtualmin module repos.