Allow building as non-root inside the sandbox

Currently all builds inside the sandbox run with UID 0. Some users would like to add a configuration option that lets you run as a non-root user inside the sandbox.

Some components fail to build if the build is executed as root. As an example, GNU Coreutils has a configure script which raises an error if you run it as root. That specific case can be worked around by setting FORCE_UNSAFE_CONFIGURE=1 in the environment. However some users are saying that they have lots more errors such as this and it is not always practical to work around them.

An additional worry is that on non-Linux platforms, we are not particularly well sandboxed currently and so having builds run as a non-root UID is desired as a safety precaution.