Default for the stage release script should not be the current user

[mattcaswell]

When running stage-release.sh if you neglect to supply the "--local-user" argument then it defaults to the current user. This is almost never the correct answer since it signs the release tarball with the current user's gpg key, rather than the team key.

It probably should default to the team key, or alternatively fail.

[levitte]

I would default it to the team key. I just found out that the key user id (i.e. any email address associated with it) can be used in place of the hexadecimal id, so it would be a good idea to use that as a default.

[levitte]

I have a few things lying around that align with this. I'll collect them into a PR on Monday.