Switching from FixedBuild to FixedBuilds (part 1).
#16412
Converting msrc files from using FixedBuild to FixedBuilds. In part 2,
FixedBuild will be completely removed.
# Checklist for submitter
If some of the following don't apply, delete the relevant line.
<!-- Note that API documentation changes are now addressed by the
product design team. -->
- [x] Changes file added for user-visible changes in `changes/` or
`orbit/changes/`.
See [Changes
files](https://fleetdm.com/docs/contributing/committing-changes#changes-files)
for more information.
- [x] Added/updated tests
