pytester: fix subprocess mode ignores all pytester.plugins except the first

As far as I can see, there is no reason for this, seems like a mistake.

Author: bluetech

changelog/13522.bugfix.rst

@@ -0,0 +1 @@
+Fixed :fixture:`pytester` in subprocess mode ignored all :attr`pytester.plugins <pytest.Pytester.plugins>` except the first.

src/_pytest/pytester.py

@@ -1494,9 +1494,9 @@ def runpytest_subprocess(
         __tracebackhide__ = True
         p = make_numbered_dir(root=self.path, prefix="runpytest-", mode=0o700)
         args = (f"--basetemp={p}", *args)
-        plugins = [x for x in self.plugins if isinstance(x, str)]
-        if plugins:
-            args = ("-p", plugins[0], *args)
+        for plugin in self.plugins:
+            if isinstance(plugin, str):
+                args = ("-p", plugin, *args)
         args = self._getpytestargs() + args
         return self.run(*args, timeout=timeout)