fix: building withing lower parallelism than dependency group size

[wesbillman]

Fixes #2042

Before this change, we would get build errors because the go.work file for a given module would include all the built modules including modules that were in the same build group layer as a given module.

For example, if we had 4 modules all in the same group [a, b, c, d] and we build with a -j2 option (2 parallel builds), we would run into an issue where a and b would build first, in parallel. a would finish and we would start to build c. But, when we looked up the schema to get shared modules for the go.work file for c, that list would now include a even though they are in the same build group and should never depend on each other.

This fix changes the build func to not re-lookup built modules and instead just use the set of builtModules that were available at the start of the build group.

With this change we should be able to safely support any -j option higher or lower than the number of modules. This probably would have eventually happened either way once we had a system with more modules than the number of cpus on a given machine.