This basically reverts 6e5165b773.
It fixes errors like
sandbox-exec: <internal init prelude>:292:47: unable to open sandbox-minimal.sb: not found
when trying to run a development Nix installed in a user's home
directory.
Also, we're trying to minimize the number of installed files
to make it possible to deploy Nix as a single statically-linked
binary.
Even with "build-use-sandbox = false", we now use sandboxing with a
permissive profile that allows everything except the creation of
setuid/setgid binaries.