fixup! sysconfig: make _sysconfigdata.py relocatable

Use `sys.base_prefix` for sysconfig relocation
Previously, we were using `sys.prefix` for relocation, which is
not right as it may change for virtualenvs. `sys.base_prefix` is
the right variable to use.

Lib/sysconfig.py

@@ -542,7 +542,7 @@ def _generate_posix_vars():
 
         for key in keys_to_replace:
             value = build_time_vars[key]
-            build_time_vars[key] = value.replace(prefix, sys.prefix)
+            build_time_vars[key] = value.replace(prefix, sys.base_prefix)
     """
 
     with open(destfile, 'w', encoding='utf8') as f: