Fix CI lint and test workflow

- Export main in test file so toolkit uses `use` resolution
  (relative to file dir) instead of `source` (fails from repo root)
- Match setup-nu version to existing CI (v3.9)

Author: hyiltiz

.github/workflows/test-auto-venv.yml

@@ -16,7 +16,7 @@ jobs:
         steps:
             - uses: actions/checkout@v4
 
-            - uses: hustcer/setup-nu@v3.23
+            - uses: hustcer/setup-nu@v3.9
               with:
                   version: "*"
                   check-latest: true

modules/virtual_environments/auto-venv/test_auto_venv.nu

@@ -4,7 +4,7 @@
 use path_extensions.nu
 use venv_helpers.nu
 
-def main [] {
+export def main [] {
     let test_root = (mktemp -d | path expand)
     mut results = []