[lldb] More reliably detect a virtual environment

More reliably detect whether the API tests are running in a virtual
environment by comparing sys.prefix and sys.base_prefix [1].

[1] https://docs.python.org/3/library/sys.html#sys.base_prefix

Author: JDevlieghere

lldb/test/API/lit.cfg.py

@@ -7,6 +7,7 @@
 import shlex
 import shutil
 import subprocess
+import sys
 
 import lit.formats
 
@@ -64,7 +65,7 @@ def find_python_interpreter():
 
     # If we're running in a virtual environment, we already have a copy of the
     # Python executable.
-    if "VIRTUAL_ENV" in config.environment:
+    if sys.prefix != sys.base_prefix:
         return None
 
     # Avoid doing any work if we already copied the binary.