To make the process of automatically releasing snaps to the store from Launchpad (and presumably other similar automated systems) more reliable, we need to keep track of the time when each snap build started. This will allow the store to ensure that older builds don't accidentally supersede newer builds, even if the push-and-release of an older build was retried after an earlier failure. As a prerequisite for this, snapcraft should record the timestamp when the build was started somewhere in snap/manifest.yaml, as an RFC3339-formatted timestamp. This timestamp should also be overrideable in some way (e.g. an environment variable) so that automated build systems that wrap snapcraft can record the start of their overall build job rather than just when snapcraft started.