Bazel on NixOS is problematic -- it's packaged, but not patched to
actually _work_ outside of FHS environments.
Vere build system presents an additional accidental hurdle: it expects
the musl-cross-make toolchain to be under /usr/local/<target> and uses
sudo to install it there, as one does. Unfortunately NixOS nerfs sudo
in user namespaces (which are used to implement FHS environments) for
security reasons, making it impossible to install the toolchain the
normal way.
So in order to restore the ability to build Vere under NixOS without
resorting to containers (which is unsporting), this commit adds a
flake, which defines a devshell, which provides the toolchain and all
the other needed tools. So one can "nix develop", go get
coffee (first invocation may take 15 minutes or so, subsequent ones
will be instant until next nix store GC), and then "bazel build
:urbit" etc.