Swap venv and virtualenv checks

Environments created by pypa/virtualenv >=20 can pass both real_prefix
and base_prefix checks, but are only able to use the pyvenv.cfg values,
not the legacy `no-global-site-packages.txt`. So we need to check for
venv (PEP 405) first.

news/7718.bugfix

@@ -0,0 +1,2 @@
+Correctly detect global site-packages availability of virtual environments
+created by PyPA’s virtualenv>=20.0.

src/pip/_internal/utils/virtualenv.py

@@ -105,11 +105,12 @@ def virtualenv_no_global():
     # type: () -> bool
     """Returns a boolean, whether running in venv with no system site-packages.
     """
+    # PEP 405 compliance needs to be checked first since virtualenv >=20 would
+    # return True for both checks, but is only able to use the PEP 405 config.
+    if _running_under_venv():
+        return _no_global_under_venv()
 
     if _running_under_regular_virtualenv():
         return _no_global_under_regular_virtualenv()
 
-    if _running_under_venv():
-        return _no_global_under_venv()
-
     return False