web: Retain previous build name in retest link

I don't know how to test this change, so it might break things, and it might not actually work. But, it seems like there is a chance that it might work. :)

If it does work, then the retest link should populate the jenkins form with the same name as was used when the job was previously run. Currently the jenkins form will be populated by the default value for the job name, which usually seems to be "manually_started".

Fixes: #6 (closed)