fix: looks for groups instead of skip_install

Signed-off-by: Frost Ming <me@frostming.com>

README.md

@@ -59,7 +59,7 @@ groups = test
 commands = test
 ```
 
-If you want to skip the pdm install for some environments, you can set `skip_install = true` in the environment config.
+If no groups are specified, the pdm install will be skipped.
 
 A real-world example can be found at this repository's [tox.ini](https://github.com/pdm-project/tox-pdm/blob/main/tox.ini) and
 [GitHub Action workflow](https://github.com/pdm-project/tox-pdm/blob/main/.github/workflows/ci.yml).

tox_pdm/plugin.py

@@ -42,11 +42,11 @@ def _setup_env(self) -> None:
         pdm = self.options.pdm
         if pdm not in self.conf["allowlist_externals"]:
             self.conf["allowlist_externals"].append(pdm)
-        if self.conf["skip_install"]:
-            return
         groups = self.conf["groups"]
         op = "sync" if self.conf["pdm_sync"] else "install"
         cmd = [pdm, op, "--no-self"]
+        if not groups:
+            return
         for group in groups:
             cmd.extend(("--group", group))
         set_env: SetEnv = self.conf["setenv"]