build-bazel-package: prefix bazel with the USER variable

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]: https://github.com/bazelbuild/bazel/blob/9323c57607d37f9c949b60e293b573584906da46/src/main/cpp/startup_options.cc#L123-L124