Bazel computes the default value of output_user_root before parsing the
flag[0]. The computation of the default value involves getting the $USER
from the environment. I don't have that variable when building with
sandbox enabled.
[0]: 9323c57607/src/main/cpp/startup_options.cc (L123-L124)
A separate function for building Bazel-bazed packages. Internally it splits the
build into two phases, fetching and building.
Users are expected to provide `fetchArgs.sha256` -- checksum of fetched
dependencies. Local dependencies should be removed in `fetchArgs.preInstall`.
Overall `fetchArgs` and `buildArgs` can be used to add specific steps to fetch
and build.
