Backport PR #12360: use $SHELL in system_piped

IPython/utils/_process_posix.py

@@ -60,9 +60,10 b' class ProcessHandler(object):'
     @property
     def sh(self):
         if self._sh is None:        
-            self._sh = pexpect.which('sh')
+            shell_name = os.environ.get("SHELL", "sh")
+            self._sh = pexpect.which(shell_name)
             if self._sh is None:
-                raise OSError('"sh" shell not found')
+                raise OSError('"{}" shell not found'.format(shell_name))
         
         return self._sh