Make feature builds available for testing/review purposes.

[zenmonkeykstop]

We currently make the following package types available:

• releases (via apt.freedom.press main component)
• release candidates (via apt-test.freedom.press main component)
• nightly builds (via apt-test.freedom.press nightlies component)

Packages are built as part of CI for feature PRs, but not retained (not counting GH Actions' default behaviour). It would be useful for testing purposes if we:

1. explicitly kept these packages
2. developed an easy way for testers to apply them to an existing SDW installation.