signindex: support signing index-v1.jar

This is a bit different than index.jar: instead of their being index.xml
and index_unsigned.jar, the presense of index-v1.json means that there is
unsigned data.  That file is then stuck into a jar and signed by the
signing process.  index-v1.json is never published to the repo.  It is
included in the binary transparency log, if that is enabled.
1 job for index-v1 in 21 minutes and 47 seconds (queued for 6 seconds)
Status Job ID Name Coverage
passed #12449681