invoking niv in some subdirectory should work

[andir]

I commonly invoke niv commands somewhere in my project dir. Those fail due to niv not finding the nix/sources.json as seen below.

project/someDir $ niv update
Updating all packages
  WARNING: Could not read nix/sources.nix
     ( nix/sources.nix: openBinaryFile: does not exist (No such file or directory) )
  FATAL: Cannot use nix/sources.json
  The sources file does not exist! You may need to run 'niv init'.
  
ERROR: ExitFailure 1

It would be nice if niv would work like git, hg, svn, … in such that it tries to find the current workspace root until a filesystem boundary is encountered.

[nmattia]

Hey @andir, would love your feedback on https://github.com/nmattia/niv/issues/272