Revisit `not-a-commit`

[David Arnold]

core/grow/newImportSignatureFor.nix

Line 8 in 0ec387f

 self = cfg.inputs.self.sourceInfo // {rev = cfg.inputs.self.sourceInfo.rev or "not-a-commit";}; 

Change not-a-commit when NixOS/nix#5385 lands and assert version requirement instead.

Maybe with sunsetting process.

[David Arnold]

This is coming in 2.17.0