Add a "staging.tmp" error message for CircleCI

[bors-ng]

Add a "staging.tmp" error message for CircleCI

[notriddle]

Recognizing that the staging.tmp branch is being built is done by the webhook controller reading the commit message (it can recognize temporary commits because bors follows a format for the messages).

However, it will not post a comment unless there's one defined for the CI system in question. That is done in Message module. The "context" line, the first argument that the function needs to accept, is the same as the one used in a bors.toml file.

[notriddle]

Since the [no ci] message has been added, and CircleCI respects that, there's no reason to do this.