activation-scripts: add unmanaged system Nix to activation path

Currently, the `bin` directory of the configured system
is embedded in the `$PATH` of activation scripts, but not
other elements of the default `environment.systemPath` like
`/nix/var/nix/profiles/default/bin` or `/usr/local/bin`. This
means that when nix-darwin is not managing the Nix installation,
activation scripts like Home Manager’s that want to look up the
system‐managed Nix can’t find it. Search for it on the entire
`environment.systemPath` and add the appropriate directory if found.

We leave the launchd `activate-system` daemon alone, because it has
erroneously referred to `@out@/sw/bin` forever and therefore never got
a Nix on the path to begin with. That’s a problem for another time.

(The more ideal solution is probably for Home Manager activation to
be driven by launchd or something, but that’s a longer‐term goal.)

tests/nix-enable.nix

@@ -10,5 +10,8 @@
 
     printf >&2 'checking for unexpected nix-daemon plist in /Library/LaunchDaemons\n'
     [[ -e ${config.out}/Library/LaunchDaemons/org.nixos.nix-daemon.plist ]] && exit 1
+
+    printf >&2 'checking for late‐bound Nix lookup in /activate\n'
+    grep nixEnvPath= ${config.out}/activate
   '';
 }