nix-instantiate: Respect NIX_NO_READONLY_DEFAULT

This allows running nix-instantiate --eval-only without performing the
evaluation in readonly mode, letting features like import from
derivation and automatic substitution of `builtins.storePath` paths work.

This is an env var for two reasons:

* For projects that make regular use of these non-readonly features, it
  will be simpler to just set the env var once
* Tools that wrap nix-instantiate (like nixops) can set the env var
  without breaking backwards compatability.