Backport #25581 by @wolfogreResolve#24789
## ⚠️ BREAKING ⚠️
Before this, `DEFAULT_ACTIONS_URL` cound be set to any custom URLs like
`https://gitea.com` or `http://your-git-server,https://gitea.com`, and
the default value was `https://gitea.com`.
But now, `DEFAULT_ACTIONS_URL` supports only
`github`(`https://github.com`) or `self`(the root url of current Gitea
instance), and the default value is `github`.
If it has configured with a URL, an error log will be displayed and it
will fallback to `github`.
Actually, what we really want to do is always make it
`https://github.com`, however, this may not be acceptable for some
instances of internal use, so there's extra support for `self`, but no
more, even `https://gitea.com`.
Please note that `uses: https://xxx/yyy/zzz` always works and it does
exactly what it is supposed to do.
Although it's breaking, I belive it should be backported to `v1.20` due
to some security issues.
Follow-up on the runner side:
- https://gitea.com/gitea/act_runner/pulls/262
- https://gitea.com/gitea/act/pulls/70
Co-authored-by: Jason Song <i@wolfogre.com>
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