Build: Remove buildsInSource

[bryphe]

flex was flexing on me with this error on OSX:

error: build failed with exit code: 1
  build log:
    # esy-build-package: building: [email protected]
    # esy-build-package: pwd: /Users/bryphe/oni2/_esy/test/store/b/flex-1.2.2-b1a3d88a
    # esy-build-package: running: 'dune' 'build' '--profile=release'
    Error: mkdir: _build: Operation not permitted
    error: command failed: 'dune' 'build' '--profile=release' (exited with 1)
    esy-build-package: exiting with errors above...
    
  building [email protected]

I believe we don't need buildsInSource for this anymore, so this removes it.