Require VCS 2020.12-SP2

VCS 2020.12-SP2 contains fixes for critical issues in our verification
environment.

Signed-off-by: Philipp Wagner <phw@lowrisc.org>

tool_requirements.py

diff --git a/tool_requirements.py b/tool_requirements.py
index 9519e11..1943533 100644
--- a/tool_requirements.py
+++ b/tool_requirements.py
@@ -34,7 +34,7 @@
         'as_needed': True
     },
     'vcs': {
-        'min_version': '2020.03-SP2',
+        'min_version': '2020.12-SP2',
         'as_needed': True
     },
     'rust': {