adds fallback behavior for non-ms shells

(cherry picked from commit 770a814)

src/poetry/utils/shell.py

@@ -78,12 +78,22 @@ def activate(self, env: VirtualEnv) -> int | None:
         if sys.platform == "win32":
             if self._name in ("powershell", "pwsh"):
                 args = ["-NoExit", "-File", str(activate_path)]
-            else:
+            elif self._name == "cmd":
                 # /K will execute the bat file and
                 # keep the cmd process from terminating
                 args = ["/K", str(activate_path)]
-            completed_proc = subprocess.run([self.path, *args])
-            return completed_proc.returncode
+            else:
+                args = None
+
+            if args:
+                completed_proc = subprocess.run([self.path, *args])
+                return completed_proc.returncode
+            else:
+                # If no args are set, execute the shell within the venv
+                # This activates it, but there could be some features missing:
+                # deactivate command might not work
+                # shell prompt will not be modified.
+                return env.execute(self._path)
 
         import shlex