nix: make System, Version, SourceProfile public

[gcurtis]

Note: most of this large diff is just moving code from internal/nix/nix[_test].go to nix/nix[_test].go.

As part of some cleanup for making the DetSys installer the default, move the code related to getting the Nix version and system to the non-internal go.jetpack.io/devbox/nix package. It also merges in some nearly-duplicate code from the search indexer's Nix code (which will eventually use this package instead).

Some of the changes taken from the indexer are:

• Calling any function or method automatically...
  • sources the Nix profile if necessary.
  • looks for Nix in some well-known places if it isn't in PATH.
• nix.Version and nix.System are cached behind a sync.Once by default.

All top-level functions map to a method on a default Nix struct, following the same pattern found in flags, slog, etc.

const Version2_12 = "2.12.0" ...
var Default = &Nix{}
func AtLeast(version string) bool
func SourceProfile() (sourced bool, err error)
func System() string
func Version() string
type Info struct{ ... }
	func (i Info) AtLeast(version string) bool
type Nix struct{ ... }
	func (n *Nix) Info() (Info, error)
	func (n *Nix) System() string
	func (n *Nix) Version() string