align `system` retrieval with `nix-build`

[DavHau]

Read the system from the instantiated derivation instead of getting it from the eval time drv attrs.

This is better in terms of correctness as it derives the system through a derivations string context, instead of relying on builder implementations to set the system attribute correctly.

This derivation for example did not build with hydra before while it builds fine with the nix repl:

:b { name="hello"; type="derivation"; drvPath = hello.drvPath; }