For example, this prevents a "kvm" build on machines that don't have
KVM.

Fixes #2012.

Disable various tests if the kernel doesn't support unprivileged user
namespaces (e.g. Arch Linux disables them) or disable them via a sysctl
(Debian, Ubuntu).

Fixes #1521
Fixes #1625

You can now say '--store /tmp/nix' instead of '--store local?root=/tmp/nix'.

This ensures that command line flags such as --builders get passed
correctly.

In particular, drop the "build-" and "gc-" prefixes which are
pointless. So now you can say

  nix build --no-sandbox

instead of

  nix build --no-build-use-sandbox

This is mostly for use in the sandbox tests, since if the Nix store is
under /build, then we can't use /build as the build directory.

This only runs on Linux because it requires a diverted store (which
uses mount/user namespaces).