Describe the bug

toJSON unnecessarily truncates floats to six significant figures.

Steps To Reproduce

nix-repl> builtins.toJSON 1.000001
"1"

nix-repl> builtins.toJSON 1048576.1
"1.04858e+06"

Expected behavior

nix-repl> builtins.toJSON 1.000001
"1.000001"

nix-repl> builtins.toJSON 1048576.1
"1048576.1"

nix-env --version output

tested 2.4 and 2.5pre20211126_55275fc

Additional context

found while investigating NixOS/nixpkgs#148824