Bug

Fixes: Infrastructure

Description

Really fixes OptProf for release/* branches, also makes all drops URLs have a single source of truth, and all other references programmatically calculate the URL. Includes other minor improvements

• The build CI's "Build Insertable" job defines variables for drop names it uses, so the variable can be shared instead of recalculated over and over.
• Stop building the VS bootstrapper. All the DartLab pipelines create their own bootstrapper, so this pipeline doesn't need to.
• Fix sending the SigningType parameter to the signing step, so test signing on the official pipeline actually works.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests N/A
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc. N/A