Use nix executables in $PATH

Use nix executables in $PATH

We started pinning the nix version back Jun 2021 over
<nix/unpack-channel.nix> becoming private.  As a side effect of pinning
(overriding) the nix version, we bundled nix with pinch so that it could
find nix even if it wasn't in $PATH.

We're now unwinding that: We no longer need a specific (ancient) version
of nix, so we're going back to using whatever is laying around in $PATH.

This *could* be a breaking change if you were depending upon pinch's
ability to find nix on its own.  It seems unlikely that there are actually
any users depending on this, so I'm not bumping the major version, but
I am rolling rolling this out in two steps in case anyone is affected &
would find it useful to have an intermediate version of pinch to pin to
while they help newest pinch find nix stuff:

  pinch 3.1.0 - Uses ancient nix_2_3 that now no longer builds
  pinch 3.2.0 - Uses whatever nix is in <nixpkgs>
  pinch 3.3.0 - Uses whatever nix is in $PATH

In case pinch's bundling of its own nix was giving anyone grief because
it *wasn't* using the one in $PATH: Sorry.  It's fixed now.  You can
unwind your hacks around this.  :)