[dv] Add clkmgr to CI and nightly
[dv] Add clkmgr to CI and nightly

Don't use VCS 2020.12-SP2-1 to run clkmgr otherwise it will hang
Use VCS 2020.12-SP2 or VCS 2020.12-SP2-2 instead

Refer to #8191
Signed-off-by: Weicai Yang <4fa8c7762ee4da95e3dee7dae2c3263ea16637f9@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan