nix config check: improve error when no nix-env

It is possible that the `nix` executable is installed but not `nix-env`
(this may be unusual but for example in Fedora we have a separate
`nix-legacy` subpackage, which includes the `nix-env` symlink).

The current error message:
```
$ nix config check --verbose
Running checks against store uri: local
[FAIL] Multiple versions of nix found in PATH:

```
when there is no nix-env in PATH is confusing.

This change makes the error message precise for the missing nix-env case.

src/nix/config-check.cc

@@ -95,7 +95,9 @@ struct CmdConfigCheck : StoreCommand
                 dirs.insert(std::filesystem::canonical(candidate).parent_path());
         }
 
-        if (dirs.size() != 1) {
+        if (dirs.empty()) {
+            return checkFail("No nix-env found in PATH.");
+        } else if (dirs.size() > 1) {
             std::ostringstream ss;
             ss << "Multiple versions of nix found in PATH:\n";
             for (auto & dir : dirs)