Fix CI for external contributions

**What are you trying to do?**
PRs from forks do not have the necessary secrets to upload packages to AWS or coverage reports Codecov. This causes builds to fail, which prevents PRs from being merged.

**What's your proposed solution?**
Somehow allow PRs to be merged regardless. Possible approaches
- For Codecov, investigate what is the recommended way to deal with external PRs.
- Let workflows succeed even when upload steps fail
- Split out Upload steps such that the existing branch protections succeed.
- Extend solution for https://github.com/canonical/multipass/issues/4042

Here's an example of how external PRs fail: https://github.com/canonical/multipass/pull/4072