[wtp-releng] Please note switch from I-builds to S-builds

[David M Williams <david_williams@xxxxxxxxxx>]

In preparation for our M3 declaration. 

One difference is these bundles will now be signed (and the build 
therefore take longer, I'd guess). 

We'll delete I-builds on 'committers area' in a day or two to avoid 
confusion, but will leave them up a bit, in case anyone needs one right 
away.