nixfmt no longer defaults to formatting stdin

[Dietr1ch]

I'm getting this line at the top of my formatted buffers,

�[33mWarning: Bare invocation of nixfmt is deprecated. Use 'nixfmt -' for anonymous stdin.�[39m

Now nixfmt - needs to be called to signal we want it to process stdin. The currently available variable to tweak things, nix-nixfmt-bin, targets just the program path, but not the arguments it gets, so it's not sufficient to temporarily fix this with a customisation.

I'm not sure how to proceed since currently people might use other formatters that may or may not respect - being stdin, so it might require a breaking change or awkward backwards supporting code until people move into recent enough tools.

[Dietr1ch]

One way to kick the can down the road would be to modify the call-process-region call and instead of passing a single buffer for the output pass separate stdout and stderr buffers ('(stdout-buf stderr-buf)).

This would at least avoid corrupting the file, but will leave a stray *nixfmt-stderr* buffer with the error until the underlying problem of calling nixfmt properly is fixed.

My fear here is that it'll go unnoticed until nixfmt starts refusing to format the buffer. It'd not be work towards fixing this, but I still don't know how we "should" handle this. I'm leaning towards breaking backwards compatibility since probably all Nix/NixOS users have fresh enough systems and we have already annoyed them with the current file corrupting issue after all, so it won't be a lot worse.

[bcc32]

nix-mode checks the nix --version to determine if some things are available. It could potentially do something similar for nixfmt to decide what the calling convention should be.