feat: Generate .Release.gpg.new and use atomic replace
There is an unnecessary window when `Release.gpg` is absent. Generate
`.Release.gpg.new` and use atomic replace to update `Release.gpg`. There
is still a window where `Release` and `Release.gpg` are out of sync.
This should be addressed in the future by atomically replacing the
complete directory.
Signed-off-by: Benjamin Drung <email address hidden>
Now that https://bugs.launchpad.net/launchpad/+bug/1856774 is fixed, we can
drop our temporary workaround and use something that's both faster and
robust against the case of packages copied from PPAs owned by private teams.