Temporarily disable CredScan

[MiYanni]

Summary

Our internal builds are failing after merging the 1ES PT migration PR. This is because the CredScan tool scans the MicroBuild signing plugin, which is installed by Arcade to do signing on our builds. Normally, the baselining files would have solved this issue. However, the MicroBuild/Plugins/nuget.config has dynamic content which causes the baselining to produce a new signature hash, and thus cannot be baselined. For the time being, we'll just disable CredScan until either:

1. Arcade is updated to use the new version of the MicroBuild plugin, where it has been adjusted to install these files into a temporary location.
2. We spent the effort to properly review the SDL tool settings, and set the scanning locations accordingly to not include these files.

I validated this works on a different branch also doing the 1ES PT migration:

• https://dnceng.visualstudio.com/internal/_build/results?buildId=2412315

[mmitche]

MicroBuild got upgraded in arcade, so this can be reverted.