Add tooltip about untracked files for nix3 commands

[imadnyc]

As someone who switches between nix2 and nix3 commands often, I was annoyed when nix build .#geballs (a package that I had just written for nixpkgs and run in the nixpkgs root on my pc) didn't work but nix-build -A geballs did. The nix3 command didn't tell me that the working tree was dirty, because obviously git didn't know about it until I had staged it, so I kept getting the errors

error: flake 'git+file:///home/dre/Projects/nixpkgs' does not provide attribute 'packages.x86_64-linux.geball', 'legacyPackages.x86_64-linux.geball' or 'geball

It would be nice if there was a simple hint message along the lines of hint: did you stage your files? that always printed. I think that though this is semi-ugly to always display this everytime a flake doesn't find an attribute, I think that this is a nice stopgap until things like #6858 and #9352 get resolved. I think it'd be helpful for both beginners and people like me who forget.

It seems that all that would need to be done is add a string to here.

nix/src/libcmd/installable-flake.cc

Lines 192 to 197 in 50cb14f

	if (res.size() == 0)
	throw Error(
	suggestions,
	"flake '%s' does not provide attribute %s",
	flakeRef,
	showAttrPaths(attrPaths));

This isn't a new suggestion by any means, but I think that it would be helpful if this got its own issue so discussion specific to just the tooltip can happen.

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